avutil/tx: Fix documentation of av_tx_uninit()

Adapt it to the actual (sane) behaviour.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavutil/tx.h

@@ -154,7 +154,7 @@ int av_tx_init(AVTXContext **ctx, av_tx_fn *tx, enum AVTXType type,
                int inv, int len, const void *scale, uint64_t flags);
 
 /**
- * Frees a context and sets ctx to NULL, does nothing when ctx == NULL
+ * Frees a context and sets *ctx to NULL, does nothing when *ctx == NULL.
  */
 void av_tx_uninit(AVTXContext **ctx);