doc: module: Fix documented type of namespace

In addition to the macros EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL(), that allow

exporting of kernel symbols to the kernel symbol table, variants of these are

available to export symbols into a certain namespace: EXPORT_SYMBOL_NS() and

EXPORT_SYMBOL_NS_GPL(). They take one additional argument: the namespace.

Please note that due to macro expansion that argument needs to be a

preprocessor symbol. E.g. to export the symbol ``usb_stor_suspend`` into the

EXPORT_SYMBOL_NS_GPL(). They take one additional argument: the namespace as a

string constant. Note that this string must not contain whitespaces.

E.g. to export the symbol ``usb_stor_suspend`` into the

namespace ``USB_STORAGE``, use::

	EXPORT_SYMBOL_NS(usb_stor_suspend, "USB_STORAGE");