Fix a plural mistake REVIEWING.rst

Reviewing the REVIEWING.rst of tempest,
I noticed a plural mistake.
The word "APIs" is plural and it shouldn't add 'a' in the front.
And there is an extra "that" before "which".
So I fix it.

Change-Id: I84d81f44314feadaebfcd4a834f854364328593b
1 file changed