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

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>

Documentation/kernel-hacking/hacking.rst

@@ -601,7 +601,7 @@ Defined in ``include/linux/export.h``
 
 This is the variant of `EXPORT_SYMBOL()` that allows specifying a symbol
 namespace. Symbol Namespaces are documented in
-``Documentation/kbuild/namespaces.rst``.
+``Documentation/core-api/symbol-namespaces.rst``.
 
 :c:func:`EXPORT_SYMBOL_NS_GPL()`
 --------------------------------
@@ -610,7 +610,7 @@ Defined in ``include/linux/export.h``
 
 This is the variant of `EXPORT_SYMBOL_GPL()` that allows specifying a symbol
 namespace. Symbol Namespaces are documented in
-``Documentation/kbuild/namespaces.rst``.
+``Documentation/core-api/symbol-namespaces.rst``.
 
 Routines and Conventions
 ========================