Apply suggestions from code review

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

Author: hkBst

compiler/rustc_error_codes/src/error_codes/E0608.md

@@ -1,13 +1,13 @@
-Indexing was attempted on a type which doesn't implement
-the `std::ops::Index` trait.
+Attempted to index a value whose type doesn't implement the
+`std::ops::Index` trait.
 
 Erroneous code example:
 
 ```compile_fail,E0608
 0u8[2]; // error: cannot index into a value of type `u8`
 ```
 
-Only types which implement the `std::ops::Index` trait,
+Only values with types that implement the `std::ops::Index` trait
 can be indexed with square brackets. Example:
 
 ```