Improve the prompt for "ssh-keygen sign" (go-gitea#28509)

Close go-gitea#28505, ref:
* go-gitea#20112 (comment)
* go-gitea#28505 (comment)

Author: wxiaoguang

templates/user/settings/keys_ssh.tmpl

@@ -78,7 +78,7 @@
 							<input readonly="" value="{{$.TokenToSign}}">
 							<div class="help">
 								<p>{{ctx.Locale.Tr "settings.ssh_token_help"}}</p>
-								<p><code>{{printf "echo -n '%s' | ssh-keygen -Y sign -n gitea -f /path_to_your_privkey" $.TokenToSign}}</code></p>
+								<p><code>{{printf "echo -n '%s' | ssh-keygen -Y sign -n gitea -f /path_to_PrivateKey_or_RelatedPublicKey" $.TokenToSign}}</code></p>
 							</div>
 							<br>
 						</div>