docs: mm: slab.h: fix a broken cross-reference

There is a typo at the cross-reference link, causing this warning:

  include/linux/slab.h:11: WARNING: undefined label: memory-allocation (if the link has no caption the label must precede a section header)
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Link: http://lkml.kernel.org/r/0aeac24235d356ebd935d11e147dcc6edbb6465c.1586359676.git.mchehab+huawei@kernel.orgSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/slab.h

@@ -501,7 +501,7 @@ static __always_inline void *kmalloc_large(size_t size, gfp_t flags)
  * :ref:`Documentation/core-api/mm-api.rst <mm-api-gfp-flags>`
  *
  * The recommended usage of the @flags is described at
- * :ref:`Documentation/core-api/memory-allocation.rst <memory-allocation>`
+ * :ref:`Documentation/core-api/memory-allocation.rst <memory_allocation>`
  *
  * Below is a brief outline of the most useful GFP flags
  *