jxl/color_encoding.h: fix documented PQ standard (#1390)
jxl/color_encoding.h: fix documented PQ standard (#1390)

Fixes a copy/paste typo so JXL_TRANSFER_FUNCTION_PQ is
properly documented as coming from SMPTE ST 2084.
libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl