Fix readme instructions

[mike4192]

Thank you for the excellent straightforward working module.

I believe there is an error in instructions, as they refer to changing permissions for tvservice when the module actually uses vcgencmd for monitor control. I think older versions of this module probably used tvservice but there is no longer any reference or use of tvservice in it currently.

TBH, however, I'm not sure if vcgencmd even needs the same permission modifications to be called, but this would clear up confusions.

This change would probably address issue #123