docs: update portainer.md (#9337)

Changed #3 to lowercase, as Portainer stacks do not accept capital letters.

docs/docs/install/portainer.md

@@ -8,7 +8,7 @@ Install Immich using Portainer's Stack feature.
 
 1. Go to "**Stacks**" in the left sidebar.
 2. Click on "**Add stack**".
-3. Give the stack a name (i.e. Immich), and select "**Web Editor**" as the build method.
+3. Give the stack a name (i.e. immich), and select "**Web Editor**" as the build method.
 4. Copy the content of the `docker-compose.yml` file from the [GitHub repository](https://github.com/immich-app/immich/releases/latest/download/docker-compose.yml).
 5. Replace `.env` with `stack.env` for all containers that need to use environment variables in the web editor.