docs: accounting: update delay-accounting.rst reference

docs: accounting: update delay-accounting.rst reference

The file name: accounting/delay-accounting.rst
should be, instead: Documentation/accounting/delay-accounting.rst.

Also, there's no need to use doc:`foo`, as automarkup.py will
automatically handle plain text mentions to Documentation/
files.

So, update its cross-reference accordingly.

Fixes: 8d4df6235e14 ("delayacct: Document task_delayacct sysctl")
Fixes: c9a00e608a6e ("docs: accounting: convert to ReST")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>