Fix diskutil deleteVolume command in Uninstall guide (#11371)

Co-authored-by: Valentin Gagarin <valentin@gagarin.work>
(cherry picked from commit 8e63dc4455)

doc/manual/src/installation/uninstall.md

@@ -141,7 +141,9 @@ which you may remove.
    diskutil list
    ```
 
-   If you _do_ find a "Nix Store" volume, delete it by running `diskutil deleteVolume` with the store volume's `diskXsY` identifier.
+   If you _do_ find a "Nix Store" volume, delete it by running `diskutil apfs deleteVolume` with the store volume's `diskXsY` identifier.
+
+   If you get an error that the volume is in use by the kernel, reboot and immediately delete the volume before starting any other process.
 
 > **Note**
 >