doc/filters: add [] labels for the graph of the opening paragraph example

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/filters.texi

@@ -9,9 +9,10 @@ To illustrate the sorts of things that are possible, we consider the
 following filtergraph.
 
 @example
+                [main]
 input --> split ---------------------> overlay --> output
             |                             ^
-            |                             |
+            |[tmp]                  [flip]|
             +-----> crop --> vflip -------+
 @end example