docs: less ambiguity in quiz3

Motivation
----------
I'm currently doing the rustlings course. I was pretty lost at quiz3 because I did not understand the problem.

With this change I would have understood that it should be possible to use both `f32` or `&str` for the grade.

I also googled quiz3 and found that other people got confused and used enums: https://www.reddit.com/r/learnrust/comments/wccx7s/rustlings_blocked_on_quiz3/

<details>
  <summary>My solution, hopefully I got it now</summary>

```
// quiz3.rs
//
// This quiz tests:
// - Generics
// - Traits
//
// An imaginary magical school has a new report card generation system written
// in Rust! Currently the system only supports creating report cards where the
// student's grade is represented numerically (e.g. 1.0 -> 5.5). However, the
// school also issues alphabetical grades (A+ -> F-) and needs to be able to
// print both types of report card!
//
// Make the necessary code changes in the struct ReportCard and the impl block
// to support alphabetical report cards. Change the Grade in the second test to
// "A+" to show that your changes allow alphabetical grades.
//
// Execute `rustlings hint quiz3` or use the `hint` watch subcommand for a hint.

// I AM NOT DONE

use std::fmt::Display;

pub struct ReportCard<T: Display> {
    pub grade: T,
    pub student_name: String,
    pub student_age: u8,
}

impl <T: Display> ReportCard<T> {
    pub fn print(&self) -> String {
        format!("{} ({}) - achieved a grade of {}",
            &self.student_name, &self.student_age, &self.grade)
    }
}

mod tests {
    use super::*;

    #[test]
    fn generate_numeric_report_card() {
        let report_card = ReportCard {
            grade: 2.1,
            student_name: "Tom Wriggle".to_string(),
            student_age: 12,
        };
        assert_eq!(
            report_card.print(),
            "Tom Wriggle (12) - achieved a grade of 2.1"
        );
    }

    #[test]
    fn generate_alphabetic_report_card() {
        // TODO: Make sure to change the grade here after you finish the exercise.
        let report_card = ReportCard {
            grade: "A+",
            student_name: "Gary Plotter".to_string(),
            student_age: 11,
        };
        assert_eq!(
            report_card.print(),
            "Gary Plotter (11) - achieved a grade of A+"
        );
    }
}
 ```

</details>

exercises/quiz3.rs

@@ -11,8 +11,7 @@
 // print both types of report card!
 //
 // Make the necessary code changes in the struct ReportCard and the impl block
-// to support alphabetical report cards. Change the Grade in the second test to
+// to support alphabetical report cards.
-// "A+" to show that your changes allow alphabetical grades.
 //
 // Execute `rustlings hint quiz3` or use the `hint` watch subcommand for a hint.
 
@@ -50,9 +49,8 @@ mod tests {
 
     #[test]
     fn generate_alphabetic_report_card() {
-        // TODO: Make sure to change the grade here after you finish the exercise.
         let report_card = ReportCard {
-            grade: 2.1,
+            grade: "A+",
             student_name: "Gary Plotter".to_string(),
             student_age: 11,
         };