fix(str.md): avoid misunderstanding that there were only two string types

With the previous wording, some might think there are exactly two string types in Rust. Even in the standard library you find more string types than `String` and `&str`, like CStr, OsStr or Path. It is also possible to define your own string types, so theoretically there is an infinite amount of string types in Rust.

Author: AndreasHuber

src/std/str.md

@@ -1,6 +1,6 @@
 # Strings
 
-There are two types of strings in Rust: `String` and `&str`.
+The two most used string types in Rust are `String` and `&str`.
 
 A `String` is stored as a vector of bytes (`Vec<u8>`), but guaranteed to
 always be a valid UTF-8 sequence. `String` is heap allocated, growable and not