Add punctuation and fix capitalization in Getting Started guide

Fix more small grammar mistakes in the [Getting Started guide](http://edgeguides.rubyonrails.org/getting_started.html#security), in sections 9 and 10.

Specifically, within sections 9.1 “Basic Authentication”, 9.2 “Other Security Considerations”, and 10 “What's Next?”.

“Rails Guides” is generally capitalized on the site, including on the linked page and in the title of the site, so I capitalized it.

“The” is not part of the name of the Ruby on Rails Security Guide – “The” is not in the title on that page.

And I added a missing period and a missing colon at the ends of two sentences.

guides/source/getting_started.md

@@ -2012,7 +2012,7 @@ class CommentsController < ApplicationController
 ```
 
 Now if you try to create a new article, you will be greeted with a basic HTTP
-Authentication challenge
+Authentication challenge:
 
 ![Basic HTTP Authentication Challenge](images/getting_started/challenge.png)
 
@@ -2027,7 +2027,7 @@ along with a number of others.
 
 Security, especially in web applications, is a broad and detailed area. Security
 in your Rails application is covered in more depth in
-The [Ruby on Rails Security Guide](security.html)
+the [Ruby on Rails Security Guide](security.html).
 
 
 What's Next?
@@ -2038,7 +2038,7 @@ update it and experiment on your own. But you don't have to do everything
 without help. As you need assistance getting up and running with Rails, feel
 free to consult these support resources:
 
-* The [Ruby on Rails guides](index.html)
+* The [Ruby on Rails Guides](index.html)
 * The [Ruby on Rails Tutorial](http://railstutorial.org/book)
 * The [Ruby on Rails mailing list](http://groups.google.com/group/rubyonrails-talk)
 * The [#rubyonrails](irc://irc.freenode.net/#rubyonrails) channel on irc.freenode.net