Fix documentation bug

The first paragraph must be separated from the next paragraph.
Otherwise, rustdoc will consider the content of the latter as part of
the title.

Author: barosl

src/libstd/os.rs

@@ -889,6 +889,7 @@ pub fn errno() -> uint {
 }
 
 /// Return the string corresponding to an `errno()` value of `errnum`.
+///
 /// # Example
 /// ```rust
 /// use std::os;