memblock: get rid of a :c:type leftover

chanseset b3a7bb1851c8 ("docs: get rid of :c:type explicit declarations for structs")
removed several :c:type: markups, except by one.

Now, Sphinx 3.x complains about it:

	.../Documentation/core-api/boot-time-mm:26: ../mm/memblock.c:51: WARNING: Unparseable C cross-reference: 'struct\nmemblock_type'
	Invalid C declaration: Expected identifier in nested name, got keyword: struct [error at 6]
	  struct
	memblock_type
	  ------^

As, on Sphinx 3.x, the right markup is c:struct:`foo`.

So, let's remove it, relying on automarkup.py to convert it.
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

mm/memblock.c

@@ -50,8 +50,8 @@
  *
  * Each region is represented by struct memblock_region that
  * defines the region extents, its attributes and NUMA node id on NUMA
- * systems. Every memory type is described by the :c:type:`struct
- * memblock_type` which contains an array of memory regions along with
+ * systems. Every memory type is described by the struct memblock_type
+ * which contains an array of memory regions along with
  * the allocator metadata. The "memory" and "reserved" types are nicely
  * wrapped with struct memblock. This structure is statically
  * initialized at build time. The region arrays are initially sized to