Fix typo error in README.rst

From source code we can see the config file should be
/etc/os-collect-config.conf instead of
/etc/os-collect-config/sources.ini

Change-Id: Ieb1149991c4009fe05610ee5737d320274597404
Tidy up README.rst, formatting

Wrap long line, add heading, formatting.

Change-Id: Id6d09c19832385a64134cd0115b909ecc7c6de29