docs: process/index.rst: Fix reference to nonexistent document

Fix the following warning:

WARNING: toctree contains reference to nonexistent document
'process/unaligned-memory-access'

The path to the document was wrong.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200718165107.625847-5-dwlsalmeida@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>