Fix syntax typo

Author: guysoft

README.rst

@@ -57,7 +57,7 @@ Requirements
 
 Known to work building configurations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-1. Using the [CustomPiOS docker image](https://hub.docker.com/r/guysoft/custompios)
+1. Using the `CustomPiOS docker image <https://hub.docker.com/r/guysoft/custompios>`_
 2. Linux (Ubuntu / Debian etc)
 3. OS X -  `See this thread for information <https://github.com/guysoft/OctoPi/issues/388#issuecomment-316327106>`_