Improve formatting on "Contributing to Gerrit" page

- Use backticks around code, commands, and file names.

- Use single quotes instead of double quotes when referring to
menu names.

- Use hyperlinks rather than bare http:// links

- Refer to "topics" rather than "topic branches".

- Fix a couple of typos.

Change-Id: Ic279a7bbe3459cfa36886a24d2c8cbfc809d9048
1 file changed