docs: Use underscore not hyphen in label

sphinx emits warning

       WARNING: undefined label: memory-allocation ...

This seems to be caused by the use of a hyphen in the label name instead
of an underscore.  Using an underscore for the label name and the
reference clears the warning.

Use underscore not hyphen in label and reference.
Signed-off-by: NTobin C. Harding <tobin@kernel.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/core-api/memory-allocation.rst

-.. _memory-allocation:
+.. _memory_allocation:
 
 =======================
 Memory Allocation Guide

Documentation/vm/index.rst

@@ -4,7 +4,7 @@ Linux Memory Management Documentation
 
 This is a collection of documents about the Linux memory management (mm)
 subsystem.  If you are looking for advice on simply allocating memory,
-see the :ref:`memory-allocation`.
+see the :ref:`memory_allocation`.
 
 User guides for MM features
 ===========================