CONTRIBUTORS: replace old e-mail address

I don't have access to this e-mail inbox anymore.

Change-Id: Ia3dff6a56c7f6c782be74a998a622ef0611eca7e
Reviewed-on: https://go-review.googlesource.com/122456Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

CONTRIBUTORS

@@ -1058,7 +1058,7 @@ Michael Matloob <matloob@google.com>
 Michael McConville <momcconville@gmail.com>
 Michael McGreevy <mcgreevy@golang.org>
 Michael McLoughlin <mmcloughlin@gmail.com>
-Michael Munday <munday@ca.ibm.com>
+Michael Munday <mike.munday@ibm.com>
 Michael Pearson <mipearson@gmail.com>
 Michael Piatek <piatek@google.com>
 Michael Pratt <mpratt@google.com>