add warn for store in password manager

.env.sample

 # FTP host (the storage box adresses for Tetras)
 host="CHANGEME"
-# The FTP user (or sub user for Tetras)
+# The FTP user (or sub user for Tetras) /!\ to put in the password manager
 user="CHANGEME"
 # FTP password (give at creation of subuser at Tetras) /!\ to put in the password manager
 ftppass="CHANGEME"