Fix doc titles

[ci skip]

docs/dns.md

@@ -1,4 +1,4 @@
-# Overview
+# DNS Configuration
 
 This is a work in progress.
 

docs/docker-options.md

@@ -1,4 +1,4 @@
-# Docker Options
+# Docker Container Options
 
 > New as of 0.3.17
 

docs/nginx.md

@@ -1,4 +1,4 @@
-# Nginx
+# Nginx Configuration
 
 Dokku uses nginx as it's server for routing requests to specific applications. By default, access and error logs are written for each app to `/var/log/nginx/${APP}-access.log` and `/var/log/nginx/${APP}-error.log` respectively