Fix a typo in YuvConveter RGB shader code.
Fix a typo in YuvConveter RGB shader code.

TBR=magjed@webrtc.org

Bug: None
Change-Id: Ia2594f6191510cb971f435ca9c27418cce929617
Reviewed-on: https://webrtc-review.googlesource.com/12920
Reviewed-by: Sami Kalliomäki <b9debeb8943b89663e00c95863751401cba91c9a@webrtc.org>
Commit-Queue: Sami Kalliomäki <b9debeb8943b89663e00c95863751401cba91c9a@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#20332}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc