Fix typo (instruciton -> instruction).

Originally committed as revision 25157 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/optimization.txt

@@ -157,7 +157,7 @@ Use asm loops like:
 __asm__(
     "1: ....
     ...
-    "jump_instruciton ....
+    "jump_instruction ....
 Do not use C loops:
 do{
     __asm__(