adding common path to modules directory

adding some detail to the instructions as I go through them. Just in case someone does not know where modules are stored. I added the path.

docs/user_guide/install_tripal/upgrade_from_tripal2.rst

@@ -44,7 +44,7 @@ Step 1: Upgrade Tripal
 
     drush pm-disable tripal_core
 
-4. The Tripal modules must also be downloaded and updated. To do this, delete the old Tripal v2 modules directories (be sure you have a backup before removing). The following command will retrieve the Tripal 3 version:
+4. The Tripal modules must also be downloaded and updated. To do this, delete the old Tripal v2 modules directories, usually found in /var/www/html/sites/all/modules (be sure you have a backup before removing). The following command will retrieve the Tripal 3 version:
 
   .. code-block:: bash