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: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>