corrected mistakes that resulted from the previous commit

 * changed back the first sentence to it's original form
 * added back a word that was deleted by mistake

Author: Kaartic Sivaraam

book/06-github/1-github.asc

@@ -2,7 +2,7 @@
 == GitHub
 
 (((GitHub)))
-GitHub is one of the largest host for Git repositories, and is the central point of collaboration for millions of developers and projects.
+GitHub is the single largest host for Git repositories, and is the central point of collaboration for millions of developers and projects.
 A large percentage of all Git repositories are hosted on GitHub, and many open-source projects use it for Git hosting, issue tracking, code review, and other things.
 So while it's not a direct part of the Git open source project, there's a good chance that you'll want or need to interact with GitHub at some point while using Git professionally.
 

book/06-github/sections/2-contributing.asc

@@ -414,4 +414,4 @@ This isn't technically GitHub Flavored Markdown, but it is incredibly useful. In
 .Drag and drop images to upload them and auto-embed them.
 image::images/markdown-08-drag-drop.png[Drag and drop images]
 
-If you at <<_md_drag>>, you can see a small ``Parsed as Markdown'' hint above the text area. Clicking on that will give you a full cheat sheet of everything you can do with Markdown on GitHub.
+If you look at <<_md_drag>>, you can see a small ``Parsed as Markdown'' hint above the text area. Clicking on that will give you a full cheat sheet of everything you can do with Markdown on GitHub.