Fix a pesky type error
```
error[E0308]: mismatched types
--> src/ch08-03-hash-maps.md:44:15
|
44 | keys of type `&str` and values of type `i32`. Like vectors, hash maps are
|               ^ expected struct `std::string::String`, found &str
|
= note: to fix, merge this pull request
```
@@ -41,7 +41,7 @@ in the prelude. Hash maps also have less support from the standard library;
there's no built-in macro to construct them, for example.
Just like vectors, hash maps store their data on the heap. This `HashMap` has
-keys of type `&str` and values of type `i32`. Like vectors, hash maps are
+keys of type `String` and values of type `i32`. Like vectors, hash maps are
homogeneous: all of the keys must have the same type, and all of the values must
have the same type.
