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 <dmitshur@golang.org>
diff --git a/devapp/owners/owners.go b/devapp/owners/owners.go
index c811943..3cb7f7f 100644
--- a/devapp/owners/owners.go
+++ b/devapp/owners/owners.go
@@ -188,12 +188,12 @@
 		<span class="path">{{$path}}</span>
 		<span class="primary">
 			{{range .Primary}}
-				<a href="https://github.com/{{.GitHubUsername}}" target="_blank" rel="noopener">{{.GitHubUsername}}</a>
+				<a href="https://github.com/{{.GitHubUsername}}" target="_blank" rel="noopener">@{{.GitHubUsername}}</a>
 			{{end}}
 		</span>
 		<span class="secondary">
 			{{range .Secondary}}
-				<a href="https://github.com/{{.GitHubUsername}}" target="_blank" rel="noopener">{{.GitHubUsername}}</a>
+				<a href="https://github.com/{{.GitHubUsername}}" target="_blank" rel="noopener">@{{.GitHubUsername}}</a>
 			{{end}}
 		</span>
 	</div>