docs: admin-guide/mm/index: Fix reference to nonexistent document

Fix the following warning:

WARNING: toctree contains reference to nonexistent document
'admin-guide/mm/nommu-map'

This was due to a typo.
Signed-off-by: NDaniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200718165107.625847-1-dwlsalmeida@gmail.comSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/mm/index.rst

@@ -31,7 +31,7 @@ the Linux memory management.
    idle_page_tracking
    ksm
    memory-hotplug
-   nommu-map
+   nommu-mmap
    numa_memory_policy
    numaperf
    pagemap