Update headings to use sentence case.

pkg/http_server/README.md

 Library of HTTP server classes.
 
 This package contains a set of high-level classes that, together with
 HttpServer, makes is easy to provide content through HTTP servers.
 
-## Virtual Directory
+## Virtual directory
 
 The VirtualDirectory class makes it possible to easy serve static content from
 the drive. It supports:
 
  *  Range-based request, making it possible to pause/resume downloads and stream
     videos.
  *  If-Modified-Since based caching.
  *  Automatic GZip-compression of content.
  *  Ability to follow links within, either throughout the system or in a jailed
     root.
  *  Optional directory listing.
 
  The following example shows how to set up a Virtual Directory of a given path
 
     var virtualDirectory = new VirtualDirectory('/var/www/');
     virtualDirectory.serve(new HttpServer('0.0.0.0', 8080));
 
 See [VirtualDirectory](
 http://api.dartlang.org/docs/http_server/VirtualDirectory.html)
 for more info about how to customize the class.
 
-## Virtual Host
+## Virtual host
 
 The VirtualHost class makes it possible to serve multiple hosts on the same
 address, by using the `Host` field of the incoming requests. It also provides
 the ability to work on wildcards for sub-domains.
 
     var virtualHost = new VirtualHost(server);
     // Filter out on a specific host
     var stream1 = virtualServer.addHost('static.myserver.com');
     // Wildcard for any other sub-domains.
     var stream2 = virtualServer.addHost('*.myserver.com');
     // Requets not matching any hosts.
     var stream3 = virtualServer.unhandled;
 
 See [VirtualHost](
 http://api.dartlang.org/docs/http_server/VirtualHost.html)
 for more information.