OWNERS: Update the doc reference (#945)

The old reference was removed in kubernetes/kubernetes@5da316d4
(Remove stubs from docs/, 2017-06-25).  Before that removal it had
been pointing at [1].  [1] is currently a stub itself, pointing at
[2], and [2] has (for now?) the real docs.

[1]: https://github.com/kubernetes/community/blob/master/contributors/devel/owners.md
[2]: https://github.com/kubernetes/community/blob/master/contributors/guide/owners.md

Author: wking

OWNERS

@@ -1,5 +1,5 @@
 # See the OWNERS file documentation:
-#  https://github.com/kubernetes/kubernetes/blob/master/docs/devel/owners.md
+#  https://github.com/kubernetes/community/blob/master/contributors/guide/owners.md
 
 approvers:
   - aaronlevy