regexp: fix link to RE2 syntax
regexp: fix link to RE2 syntax

Fixes #10224

Change-Id: I21037379b4667575e51ab0b6b683138c505c3f68
Reviewed-on: https://go-review.googlesource.com/7960
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

matloob/regexp,matloob/regexp,matloob/regexp