dev-manual: update reference to sstate-cache-management script

* Script is now .py
* In the example, we would most likely be within the build directory
  already, so the path would be just "sstate-cache" not
  "build/sstate-cache"

(From yocto-docs rev: a655ef8f5fee98e55277cbd40a092fcae697cbae)

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/disk-space.rst

@@ -49,7 +49,7 @@ requires a full build environment to be available and doesn't work well
 covering multiple releases. It won't work either on limited environments
 such as BSD based NAS::
 
-   sstate-cache-management.sh --remove-duplicated --cache-dir=build/sstate-cache
+   sstate-cache-management.py --remove-duplicated --cache-dir=sstate-cache
 
 This command will ask you to confirm the deletions it identifies.
 Run ``sstate-cache-management.sh`` for more details about this script.