IO Docs: Fix Link in Cursor description

The first paragraph of the docs of the Cursor struct contains a Markdown
link. In listings, this won't get rendered. (Rustdoc seems to split off the
first paragraph and after that convert Markdown to HTML.)

Author: killercup

src/libstd/io/cursor.rs

@@ -15,10 +15,8 @@ use cmp;
 use io::{self, SeekFrom, Error, ErrorKind};
 use slice;
 
-/// A `Cursor` wraps another type and provides it with a [`Seek`][seek]
-/// implementation.
-///
-/// [seek]: trait.Seek.html
+/// A `Cursor` wraps another type and provides it with a
+/// [`Seek`](trait.Seek.html) implementation.
 ///
 /// Cursors are typically used with in-memory buffers to allow them to
 /// implement `Read` and/or `Write`, allowing these buffers to be used