Documentation: kernel-hacking: hacking.rst: Change reference to document namespaces.rst to symbol-namespaces.rst

This patch fixes the following documentation build warning:
Warning: Documentation/kernel-hacking/hacking.rst references
a file that doesn't exist: Documentation/kbuild/namespaces.rst

According to the following patch:
https://patchwork.kernel.org/patch/11178727/
(doc: move namespaces.rst from kbuild/ to core-api/)

The file namespaces.rst was moved from kbuild to core-api
and renamed to symbol-namespaces.rst.
Therefore, this patch changes the reference to the document
kbuild/namespaces.rst in hacking.rst to
core-api/symbol-namespaces.rst
Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
Link: https://lore.kernel.org/r/20191204104554.9100-1-madhuparnabhowmik04@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>