Fix installing error

Fix installing error below ：
Error: None of the items in $PATH are writable. Run with sudo or add a $PATH item that you have access to.

content/docs/installation/contents.lr

@@ -46,6 +46,10 @@ get Lektor installed with our installation script:
 ```
 $ curl -sf https://www.getlektor.com/install.sh | sh
 ```
+If you are not logged in as superuser, instead you should try this:
+```
+$ curl -sf https://www.getlektor.com/install.sh | sudo sh
+```
 If you would like to install Lektor without being prompted, set LEKTOR_SILENT before running the prior command.
 
 For Windows you can use the `command prompt`: