docs: look for sphinx-pre-install in the source tree

Recent makefile changes included an invocation of
./scripts/sphinx-pre-install.  Unfortunately, that fails when a separate
build directory is in use with:

  /bin/bash: ./scripts/sphinx-pre-install: No such file or directory

Use $(srctree) to fully specify the location of this script.

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