doc/filters: Clarify scale2ref example

Signed-off-by: Kevin Mark <kmark937@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 114e871621)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/filters.texi

@@ -11915,7 +11915,7 @@ uses the reference video instead of the main input as basis.
 
 @itemize
 @item
-Scale a subtitle stream to match the main video in size before overlaying
+Scale a subtitle stream (b) to match the main video (a) in size before overlaying
 @example
 'scale2ref[b][a];[a][b]overlay'
 @end example