fix return type for getRefreshTokenGrant phpdoc#13

BenoitDuffez wants to merge 1 commit intoBenjaminFavre:masterBenjaminFavre/oauth2-http-client:masterfrom

BenoitDuffez:BenoitDuffez/fix-phpdocBenoitDuffez/oauth2-http-client:BenoitDuffez/fix-phpdocCopy head branch name to clipboard