doc/filter_design: fix a few other typo.

doc/filter_design.txt

@@ -59,9 +59,9 @@ Frame scheduling
     filter; these buffered frames must be flushed immediately if a new input
     produces new output.
 
-    ( Example: framerate-doubling filter: start_frame must (1) flush the
-    second copy of the previous frame, it it is stille there, (2) push the
-    first copy of the incoming frame, (3) keep the second copy for later. )
+    (Example: framerate-doubling filter: start_frame must (1) flush the
+    second copy of the previous frame, if it is still there, (2) push the
+    first copy of the incoming frame, (3) keep the second copy for later.)
 
     If the input frame is not enough to produce output, the filter must not
     call request_frame to get more. It must just process the frame or queue
@@ -89,7 +89,7 @@ Frame scheduling
     Return values:
     if request_frame could produce a frame, it should return 0;
     if it could not for temporary reasons, it should return AVERROR(EAGAIN);
-    if it could not because there are no more frames, it shous return
+    if it could not because there are no more frames, it should return
     AVERROR_EOF.
 
     The typical implementation of request_frame for a filter with several