docs(prerequisites): add `--no-save` to `npm install`

To avoid the mistake fixed by the previous commit, ensure users install
the Node.js dependencies without polluting the lock file by passing
`--no-save` to the `npm install` line.

Change-Id: I10b5cc17b9001fc2e26deee02bf99ce033a949c1
Signed-off-by: Chris Kay <chris.kay@arm.com>

docs/getting_started/prerequisites.rst

@@ -144,7 +144,7 @@ running from within your newly-cloned repository:
 
 .. code:: shell
 
-    npm install
+    npm install --no-save
 
 If you have opted **not** to install Node.js, you can install the Gerrit hook
 manually by running: