base64: fix little typo

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/base64.c

@@ -187,7 +187,7 @@ int main(void)
     for(i=0; i<10000; i++){
         START_TIMER
         av_base64_decode(NULL, out, 0);
-        STOP_TIMER("sytax check")
+        STOP_TIMER("syntax check")
     }
 
     return error_count;