config/format.txt: specify default value of format.coverLetter

Signed-off-by: Denton Liu <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

Documentation/config/format.txt

@@ -77,6 +77,7 @@ format.coverLetter::
 	A boolean that controls whether to generate a cover-letter when
 	format-patch is invoked, but in addition can be set to "auto", to
 	generate a cover-letter only when there's more than one patch.
+	Default is false.
 
 format.outputDirectory::
 	Set a custom directory to store the resulting files instead of the