Fix path to LightTable (for Ubuntu setup)

In order to launch LightTable as described, PATH should contain the path to the executable
All mention of setting the PATH in these instructions follows this style - i.e. setting it at the command line (and for the duration of the session). If, as suggested, we were to switch to setting the PATH in config, we would have to do this across all instructions for discovering which shell is being used and then covering instructions for several flavours of shell (zsh, bash). I suspect that in order to avoid the complexity that this introduces, the instructions just suggest exporting the PATH variable at the command line.