doc/scaler, swscale/options: use proper capitalization

Proper names should be capitalized in all user facing API as far as
possible. The option names themselves have not been changed since:
1. We consistently keep option names in lower case.
2. Changing them would break existing scripts.

The converse is also true: improper names should not be capitalized
generally.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
