Update doc and fix the path of msmtp.
--- a/doc/Mutt+msmtp.txt
+++ b/doc/Mutt+msmtp.txt
@@ -40,7 +40,7 @@
 
    Add the following lines to your Mutt configuration file:
 
-     set sendmail="/usr/local/bin/msmtp"
+     set sendmail="/usr/bin/msmtp"
      set from=you@example.com
 
 
