Fix a typo in the description of CSP_UNKNOWN

Change "source video transfer function" to "chroma sample position" in
the description of CSP_UNKNOWN.

Fixes AOMediaCodec#309.

07.bitstream.semantics.md

@@ -458,7 +458,7 @@ that subsampling_x is equal to 0 and subsampling_y is equal to 0.
 
 | chroma_sample_position | Name of chroma sample position| Description
 |:----------------------:|:------------------------------: -----------
-| 0                      | CSP_UNKNOWN                   | Unknown (in this case the source video transfer function must be signaled outside the AV1 bitstream)
+| 0                      | CSP_UNKNOWN                   | Unknown (in this case the chroma sample position must be signaled outside the AV1 bitstream)
 | 1                      | CSP_VERTICAL                  | Horizontally co-located with (0, 0) luma sample, vertical position in the middle between two luma samples
 | 2                      | CSP_COLOCATED                 | co-located with (0, 0) luma sample
 | 3                      | CSP_RESERVED                  |