Fix docs site main branch links

_includes/nav.html

@@ -1,7 +1,7 @@
 <ul>
   <li><a href="/">Home</a></li>
   <li><a href="/blog">Blog</a></li>
-  <li><a href="/docs/master">Docs</a>
+  <li><a href="/docs/main">Docs</a>
     <ul>
       {% for version in site.docs.versions %}
         <li><a href="/docs/{{ version }}">{{ version }}</a></li>

index.html

@@ -11,7 +11,7 @@ <h2>The open source, self-hosted error catcher</h2>
   just point the airbrake gem to your Errbit server.
 </p>
 <p>
-  Documentation is available for all released versions of Errbit and master. It
+  Documentation is available for all released versions of Errbit and main. It
   is built directly from whatever documentation was available in the ./docs
   folder at the time of release.
 </p>

lib/doc_builder.rb

@@ -96,7 +96,7 @@ def delete_entries(entries)
     end
   end
 
-  # master first, then descending numerically (by semver semantics)
+  # main first, then descending numerically (by semver semantics)
   def sort_versions
     @versions.sort! do |a,b|
       if a == "main"