docs(README): clarify apps_paths migration instructions

Combine steps 3.2 and 3.3 under "Migrating an existing installation"
into a single "replace" instruction. The previous two-step approach
("delete your apps_paths" then "add this apps_paths") was ambiguous
and could be misread as requiring no apps_paths configuration at all
after step 3.2.

The revised step explains that any existing apps_paths configuration
should be replaced with the Docker-compatible version, notes that the
existing configuration may vary in form, and briefly explains why the
apps directory must be non-writable and custom_apps writable.

Signed-off-by: Josh <josh.t.richards@gmail.com>

README.md

@@ -738,6 +738,7 @@ You're already using Nextcloud and want to switch to docker? Great! Here are som
     docker compose exec db sh -c "psql -U USER --set ON_ERROR_STOP=on nextcloud < /dmp"
     docker compose exec db rm /dmp
     ```
+3. Edit your config.php
 3. Edit your config.php
     1. Set database connection
         - In case of MySQL database
@@ -748,17 +749,7 @@ You're already using Nextcloud and want to switch to docker? Great! Here are som
         ```php
         'dbhost' => 'db:5432',
         ```
-    2. Make sure you have no configuration for the `apps_paths`. Delete lines like these
-        ```php
-        'apps_paths' => array (
-            0 => array (
-                'path' => OC::$SERVERROOT.'/apps',
-                'url' => '/apps',
-                'writable' => true,
-            ),
-        ),
-        ```
-    3. Make sure to have the `apps` directory non writable and the `custom_apps` directory writable
+    2. Replace any existing `apps_paths` configuration with the following Docker-compatible version. Your existing configuration may look different (e.g. using `OC::$SERVERROOT.'/apps'` or having only a single entry), but it must be replaced with exactly this to ensure shipped apps in `apps` are non-writable and user-installed apps go to `custom_apps`:
         ```php
         'apps_paths' => array (
           0 => array (
@@ -773,7 +764,7 @@ You're already using Nextcloud and want to switch to docker? Great! Here are som
           ),
         ),
         ```
-    4. Make sure your data directory is set to /var/www/html/data
+    3. Make sure your data directory is set to /var/www/html/data
         ```php
         'datadirectory' => '/var/www/html/data',
         ```