Use correct capitalization.

Not great to mix pull requests like this, but it's minor so hopefully you won't
mind.

Author: Michael C. Harris

doc/gist-vim.txt

@@ -246,9 +246,9 @@ be kept for later use.  You can revoke the token at any time from the list of
 "Authorized applications" on Github's "Account Settings" page.
 (https://github.com/settings/applications)
 
-If you have two-factor authentication enabled on Github, you'll see the message
+If you have two-factor authentication enabled on GitHub, you'll see the message
 "Must specify two-factor authentication OTP code." In this case, you need to
-create a "Personal Access Token" on Github's "Account Settings" page
+create a "Personal Access Token" on GitHub's "Account Settings" page
 (https://github.com/settings/applications) and place it in a file 
 named ~/.gist-vim like this:
 >