avisynth: Fix a "AVISynth" vs. "AviSynth" struct name typo

Also fix some similar typos in comments and documentation.

doc/general.texi

@@ -136,7 +136,7 @@ library:
 @item Apple HTTP Live Streaming @tab   @tab X
 @item ASF                       @tab X @tab X
 @item AVI                       @tab X @tab X
-@item AVISynth                  @tab   @tab X
+@item AviSynth                  @tab   @tab X
 @item AVS                       @tab   @tab X
     @tab Multimedia format used by the Creature Shock game.
 @item Beam Software SIFF        @tab   @tab X

libavfilter/vf_gradfun.c

@@ -25,7 +25,7 @@
  * libmpcodecs/vf_gradfun.c
  *
  * Apply a boxblur debanding algorithm (based on the gradfun2db
- * Avisynth filter by prunedtree).
+ * AviSynth filter by prunedtree).
  * Foreach pixel, if it's within threshold of the blurred value, make it closer.
  * So now we have a smoothed and higher bitdepth version of all the shallow
  * gradients, while leaving detailed areas untouched.

libavformat/avisynth.c

@@ -36,7 +36,7 @@ typedef struct {
 
 typedef struct {
   PAVIFILE file;
-  AVISynthStream *streams;
+  AviSynthStream *streams;
   int nb_streams;
   int next_stream;
 } AviSynthContext;