completion: fix misspelled config key aliasesfiletype

The correct name in git-send-email.perl is aliasfiletype [1]. There are
actually two instances of this misspelling. The other was found and
fixed in 6068ac8848 (completion: add missing configuration variables -
2010-12-20)

[1] 994d6c66d3 (send-email: address expansion for common mailers - 2006-05-14)

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

contrib/completion/git-completion.bash

@@ -2177,7 +2177,7 @@ _git_config ()
 		__gitcomp "$__git_log_date_formats"
 		return
 		;;
-	sendemail.aliasesfiletype)
+	sendemail.aliasfiletype)
 		__gitcomp "mutt mailrc pine elm gnus"
 		return
 		;;