devapp/owners: add @ sign before github usernames
devapp/owners: add @ sign before github usernames

For easier copy/pasting into GitHub issues.

Change-Id: I924b3e7a930dfe322594640b291af2baf3176d5f
Reviewed-on: https://go-review.googlesource.com/c/build/+/179417
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build