Corrected custom file path change

Readme tells to set custom variables with
(custom-set-variables
  '(eclim-eclipse-dirs '("~/opt/eclipse")
but should be set like this:
(custom-set-variables
'(eclim-eclipse-dirs "path-to-dir"))
(this leads to the problem that this person had: https://github.com/senny/emacs-eclim/issues/104 )
Also, it is quite misleading to put a tilde before the opt, as most people probably installs eclipse in /opt (not ~/opt).

And lastly, if specifying the eclipse path is supposed to make emacs-eclim find the eclim-executable automatically, I believe it doesn't work, therefore I added setting both variables in the readme.