Corrected typo in the headers section of the request options docs.
Corrected typo in the headers section of the request options docs.

guzzle/guzzle,Brunty/guzzle,michabbb-backup/guzzle,kkopachev/guzzle,michabbb-backup/guzzle,Brunty/guzzle,kkopachev/guzzle,guzzle/guzzle