Update Install_mail.md

Install_mail.md

-** Install: **
+#Install:
 >sudo apt install mailutils postfix
 
 >sudo DEBIAN_PRIORITY=low apt install postfix
 
-** In configuration: **
+#In configuration:
 * General type of mail configuration?: Internet Site
 * System mail name: example.com (not mail.example.com)
 * Root and postmaster mail recipient: The username of your primary Linux account