Minor typos and edits to "Credentials in Git repos"

Fix some minor typos and edits to "Credentials in Git repos".

1. Change ' to ’
2. Remove "and" in "from another background and you…"
3. Typo "if" instead of "of"
4. Typo "been" to "be" in "The files containing credentials should be
added to"
5. Markdown: Double blank space at the end of a line makes a <br> if you didn't know.

Just curious, is there are reason there was breaks in the middle of sentences?