zshrc: use "ctrl-x ." instead ",." to enable power completion / abbreviation expansion

zshrc: use "ctrl-x ." instead ",." to enable power completion / abbreviation expansion

The use of ",." for enabling the power completion /
abbreviation expansion / buffer expansion was a bad idea
because you'll get either a delay or even missing characters,
e.g. when ",." is part of a copy/paste action. So switch to
a safer approach by using "ctrl-x ." instead.