docs: mei.rst: fix a C expression markup

Sphinx 3.x doesn't allow expressions using :c:func markup:

	./Documentation/driver-api/mei/mei.rst:41: WARNING: Unparseable C cross-reference: 'close(int fd)'
	Invalid C declaration: Expected end of definition. [error at 5]
	  close(int fd)
	  -----^

So, convert it into a :c:expr.
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/driver-api/mei/mei.rst

@@ -38,7 +38,7 @@ Because some of the Intel ME features can change the system
 configuration, the driver by default allows only a privileged
 user to access it.
 
-The session is terminated calling :c:func:`close(int fd)`.
+The session is terminated calling :c:expr:`close(fd)`.
 
 A code snippet for an application communicating with Intel AMTHI client: