signal Merge Request #3: Fix compatibility with Octave 7 (open)

Octave 7.0.1 prints the following error when trying to install signal:

error: parse error near line 123 of file /usr/local/share/octave/packages/signal-1.4.1/buffer.m

  off: invalid use of symbol as both variable and command

>>>             off --;
               ^

This PR fixes the error by removing the space before the '--'.