regexp: suggest go doc, not godoc

In 1.6, go doc is more likely to be available.

Change-Id: I970ad1d3317b35273f5c8d830f75713d3570c473
Reviewed-on: https://go-review.googlesource.com/10518Reviewed-by: Andrew Gerrand <adg@golang.org>

src/regexp/regexp.go

@@ -9,7 +9,7 @@
 // More precisely, it is the syntax accepted by RE2 and described at
 // https://golang.org/s/re2syntax, except for \C.
 // For an overview of the syntax, run
-//   godoc regexp/syntax
+//   go doc regexp/syntax
 //
 // The regexp implementation provided by this package is
 // guaranteed to run in time linear in the size of the input.