Update Install.md

mkdir flag `--parents` added for idempotency.

doc/Install.md

@@ -79,7 +79,7 @@ The Linux commands to clone the repository into a directory "mywebsite" would be
 
 Make sure the folder *view/smarty3* exists and is writable by the webserver user, in this case *www-data*
 
-    mkdir view/smarty3
+    mkdir -p view/smarty3
     chown www-data:www-data view/smarty3
     chmod 775 view/smarty3