fix typo in colors section of README.md

[kvirani]

BEFORE:

If 256 or 88 colors is not supported. Blessed with reduce the color to whatever is available.

NOW:

If 256 or 88 colors are not supported, Blessed will reduce the color to whatever is available.

[ulcuber]

@kvirani I will apply this fix there: https://github.com/ulcuber/blessed