Remove array/slice syntax from argument &mut str in TODO comment for lowercase func (mainmatter#99)

Co-authored-by: thomasgrbic <[email protected]>

Author: tomgrbz

exercises/06_ticket_management/11_mutable_slices/src/lib.rs

@@ -1,6 +1,6 @@
 // TODO: Define a function named `lowercase` that converts all characters in a string to lowercase,
 //  modifying the input in place.
-//  Does it need to take a `&mut String`? Does a `&mut [str]` work? Why or why not?
+//  Does it need to take a `&mut String`? Does a `&mut str` work? Why or why not?
 
 #[cfg(test)]
 mod tests {