Fix more typos
There was a missing period and the word "features" instead of "feature" in the section about `const extern fn`s.
@@ -35,7 +35,7 @@ libc = "0.2"
This feature derives `Debug`, `Eq`, `Hash`, and `PartialEq`.
* `const-extern-fn`: Changes some `extern fn`s into `const extern fn`s.
- This features requires a nightly rustc
+ This feature requires a nightly rustc.
* **deprecated**: `use_std` is deprecated, and is equivalent to `std`.
