fix a typo in WebPPictureYUVAToARGB's doc
fix a typo in WebPPictureYUVAToARGB's doc

method -> colorspace

Change-Id: I5c9a2ccc909c967a936758dde2cfce92eb95462a

webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp