Mention ram limits

We do have that setting in the installation instructions, but we never explain what it's for, so people don't see it / don't copy it.

content/en/docs/install/faqs.md

@@ -53,7 +53,9 @@ First of all, you need to generate a pair of so-called "VAPID" keys.
 One way to do that is, from your Sharkey directory (git clone, or
 inside the Docker image):
 
-    ./packages/backend/node_modules/.bin/web-push generate-vapid-keys
+```bash
+./packages/backend/node_modules/.bin/web-push generate-vapid-keys
+```
 
 Alternatively, you can use [an online
 generator](https://www.stephane-quantin.com/en/tools/generators/vapid-keys).