runtime: fix comment

runtime: fix comment

IRIW requires 4 threads: first writes x, second writes y,
third reads x and y, fourth reads y and x.
This is Peterson/Dekker mutual exclusion algorithm based on
critical store-load sequences:
http://en.wikipedia.org/wiki/Dekker's_algorithm
http://en.wikipedia.org/wiki/Peterson%27s_algorithm

Change-Id: I30a00865afbe895f7617feed4559018f81ff4528
Reviewed-on: https://go-review.googlesource.com/7561
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>