docs: symbol-namespaces: fix reST warning with literal block

Use a literal block for the EXPORT_SYMBOL_GPL_FOR_MODULES() example to
avoid a Docutils warning about unmatched '*'. This ensures correct rendering
and keeps the source readable.

Warning:
Documentation/core-api/symbol-namespaces.rst:90: WARNING: Inline emphasis start-string without end-string. [docutils]

Signed-off-by: Khaled Elnaggar <khaledelnaggarlinux@gmail.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>