Minor typos and edits to "Credentials in Git repos"

* Change ' to ’
* Remove "and" in "from another background and you…"
* Typo "if" instead of "of"
* Typo "been" to "be" in "The files containing credentials should be
added to"
Remove breaks
Markdown: Double blank space at the end of a line makes a <br>
Merge pull request #1 from lexi/master

Minor typos and edits to "Credentials in Git repos"