regexp: fix doc comment

Fixes #2432.

R=r, r
CC=golang-dev
https://golang.org/cl/5376041

src/pkg/regexp/regexp.go

 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package regexp implements a simple regular expression library.
+// Package regexp implements regular expression search.
 //
 // The syntax of the regular expressions accepted is the same
 // general syntax used by Perl, Python, and other languages.