config.txt: move sendemail-config.txt to config/

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -403,7 +403,7 @@ include::config/rerere.txt[]
 
 include::config/reset.txt[]
 
-include::sendemail-config.txt[]
+include::config/sendemail.txt[]
 
 sequence.editor::
 	Text editor used by `git rebase -i` for editing the rebase instruction file.