dev-manual/start.rst: added missing command in Optimize your VHDX file using DiskPart

  After compact vsdisk you have to detach it before exiting
  otherwise the vdisk remains attached.

  DISKPART> select vdisk file="<path_to_VHDX_file>"
  DISKPART> attach vdisk readonly
  DISKPART> compact vdisk
  DISKPART> detach     <------------ new missing command
  DISKPART> exit

(From yocto-docs rev: bf855ecaf4bec4cef9bbfea2e50caa65a8339828)

Signed-off-by: Marco Cavallini <m.cavallini@koansoftware.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 1cc65ddf1a074f61fe5a63d222f3079b7fcb4c1e)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/dev-manual/start.rst

@@ -543,6 +543,7 @@ your Yocto Project build host:
          DISKPART> select vdisk file="<path_to_VHDX_file>"
          DISKPART> attach vdisk readonly
          DISKPART> compact vdisk
+         DISKPART> detach
          DISKPART> exit
 
 .. note::