Fix broken oauth2 authentication source edit page (go-gitea#18412)

It appears that there was a broken merge of the edit.tmpl page during the merge
of go-gitea#16594 - I am not entirely sure how this happened as the PR was correct.

This PR fixes the broken template.

Fix go-gitea#18388

Signed-off-by: Andrew Thornton <art27@cantab.net>

templates/admin/auth/edit.tmpl

@@ -286,6 +286,10 @@
 							<input id="skip_local_two_fa" name="skip_local_two_fa" type="checkbox" {{if $cfg.SkipLocalTwoFA}}checked{{end}}>
 							<p class="help">{{.i18n.Tr "admin.auths.skip_local_two_fa_helper"}}</p>
 						</div>
+					</div>
+					<div class="oauth2_use_custom_url inline field">
+						<div class="ui checkbox">
+							<label><strong>{{.i18n.Tr "admin.auths.oauth2_use_custom_url"}}</strong></label>
 							<input id="oauth2_use_custom_url" name="oauth2_use_custom_url" type="checkbox" {{if $cfg.CustomURLMapping}}checked{{end}}>
 						</div>
 					</div>