README: Mention that the gui example script must be named "lsi".

README: Mention that the gui example script must be named "lsi".

By default, the prefix for the gsu action handlers is the basename
of the script and an underscore character. In the example script of
README.md all handlers are prefixed with "lsi_", so tell the reader
that the script will only work if it is saved under the name "lsi".