docs: abi-testing.rst: enable --rst-sources when building docs

Now that ABI/testing documents were fixed, add --rst-sources to
the ABI/testing too.

Acked-by: Jonathan Corbet <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/924247[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/admin-guide/abi-testing.rst

···
 developers can easily notify them if any changes occur.
 
 .. kernel-abi:: $srctree/Documentation/ABI/testing
+   :rst: