man: don't place nginx socket in /tmp (#8757)

First of all, it's frickin' ugly and wrong, as IPC sockets should be
placed in /run and definitely not under a guessable name under
world-writable /tmp. Secondly, it can't even work as we set
PrivateTmp=yes on the service.

Hence, let's clean up the example, and simply use a socket in /run
instead.

Fixes: #8419

man/systemd-socket-proxyd.xml

@@ -111,7 +111,7 @@ Requires=proxy-to-nginx.socket
 After=proxy-to-nginx.socket
 
 [Service]
-ExecStart=/usr/lib/systemd/systemd-socket-proxyd /tmp/nginx.sock
+ExecStart=/usr/lib/systemd/systemd-socket-proxyd /run/nginx/socket
 PrivateTmp=yes
 PrivateNetwork=yes]]></programlisting>
       </example>
@@ -120,7 +120,7 @@ PrivateNetwork=yes]]></programlisting>
         <programlisting>
 <![CDATA[[…]
 server {
-    listen       unix:/tmp/nginx.sock;
+    listen       unix:/run/nginx/socket;
     […]]]>
 </programlisting>
       </example>