Update mail-proxy.md (#487)

Removed gerund from title according to style guide.

Author: theJorge

content/nginx/admin-guide/mail-proxy/mail-proxy.md

@@ -2,7 +2,7 @@
 description: Simplify your email service and improve its performance with NGINX or
   F5 NGINX Plus as a proxy for the IMAP, POP3, and SMTP protocols
 docs: DOCS-423
-title: Configuring NGINX as a Mail Proxy Server
+title: Configure NGINX as a Mail Proxy Server
 toc: true
 weight: 100
 type: