linker: clarify CONFIG_KOBJECT_TEXT_AREA too small message

The assert error message when CONFIG_KOBJECT_TEXT_AREA is
too small is confusing. Probably the original idea is for
the linker to substitue CONFIG_KOBJECT_TEXT_AREA with
the actual value. However, linker does not do that.
So change the message to say that the kconfig value needs
to be increased.

Fixes #34387

Signed-off-by: Daniel Leung <daniel.leung@intel.com>