shallow defaults to 'True' not '1' (closes #21355)

Patch by Diana Clarke.
diff --git a/Lib/filecmp.py b/Lib/filecmp.py
index dd5a2af..e5ad839 100644
--- a/Lib/filecmp.py
+++ b/Lib/filecmp.py
@@ -36,7 +36,7 @@
     f2 -- Second file name
 
     shallow -- Just check stat signature (do not read the files).
-               defaults to 1.
+               defaults to True.
 
     Return value: