libavcodec/blockdsp : fix comment. clear_block need 32 aligned now

libavcodec/blockdsp.h

@@ -33,8 +33,8 @@ typedef void (*op_fill_func)(uint8_t *block /* align width (8 or 16) */,
                              uint8_t value, ptrdiff_t line_size, int h);
 
 typedef struct BlockDSPContext {
-    void (*clear_block)(int16_t *block /* align 16 */);
-    void (*clear_blocks)(int16_t *blocks /* align 16 */);
+    void (*clear_block)(int16_t *block /* align 32 */);
+    void (*clear_blocks)(int16_t *blocks /* align 32 */);
 
     op_fill_func fill_block_tab[2];
 } BlockDSPContext;