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.
Merge pull request #15963 from roryokane/getting-started-guide-fix-grammar

Add punctuation and fix capitalization in Getting Started guide