Remove function invokation parens from documentation links.

This was never established as a convention we should follow in the 'More
API Documentation Conventions' RFC:

https://github.com/rust-lang/rfcs/blob/master/text/1574-more-api-documentation-conventions.md

src/libstd/primitive_docs.rs

@@ -420,7 +420,7 @@ mod prim_slice { }
 /// # Representation
 ///
 /// A `&str` is made up of two components: a pointer to some bytes, and a
-/// length. You can look at these with the [`.as_ptr()`] and [`len()`] methods:
+/// length. You can look at these with the [`.as_ptr`] and [`len`] methods:
 ///
 /// ```
 /// use std::slice;
@@ -447,8 +447,8 @@ mod prim_slice { }
 /// assert_eq!(s, Ok(story));
 /// ```
 ///
-/// [`.as_ptr()`]: #method.as_ptr
-/// [`len()`]: #method.len
+/// [`.as_ptr`]: #method.as_ptr
+/// [`len`]: #method.len
 ///
 /// Note: This example shows the internals of `&str`. `unsafe` should not be
 /// used to get a string slice under normal circumstances. Use `.as_slice()`