docu: add flite ffmpeg example

Signed-off-by: rogerdpack <rogerpack2005@gmail.com>

doc/filters.texi

@@ -1137,6 +1137,12 @@ Read the specified text selecting the @code{slt} voice:
 flite=text='So fare thee well, poor devil of a Sub-Sub, whose commentator I am':voice=slt
 @end example
 
+@item
+Input text to ffmpeg:
+@example
+ffmpeg -f lavfi -i flite=text='So fare thee well, poor devil of a Sub-Sub, whose commentator I am':voice=slt
+@end example
+
 @item
 Make @file{ffplay} speech the specified text, using @code{flite} and
 the @code{lavfi} device: