Add missing words in Infallible docs

This sentence was previously incomplete.

library/core/src/convert/mod.rs

@@ -662,7 +662,7 @@ impl AsMut<str> for str {
 ///
 /// However there is one case where `!` syntax can be used
 /// before `!` is stabilized as a full-fledged type: in the position of a function’s return type.
-/// Specifically, it is possible implementations for two different function pointer types:
+/// Specifically, it is possible to have implementations for two different function pointer types:
 ///
 /// ```
 /// trait MyTrait {}