textobj-between: fix typo

Signed-off-by: Takeshi Banse <takebi@laafc.net>
textobj-between: change the keybind 'F' to 'f'

Ugh, I had misread the textobj-between docs. I had iterpreted 'F' as
'f' somehow.

Thank you very much pointing me on #2, wellle!

Signed-off-by: Takeshi Banse <takebi@laafc.net>