regexp: add runnable example to regex.Split

regexp: add runnable example to regex.Split

The existing comment for regex.Split contains a plain text example,
while many of the other regex functions have runnable examples. This
change provides a runnable example for Split.

Change-Id: I5373f57f532fe843d7d0adcf4b513061ec797047
Reviewed-on: https://go-review.googlesource.com/14737
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>