doc: contributing: remove named anchor #presubmit
doc: contributing: remove named anchor #presubmit

mdbook apparently does not support this syntax and just includes the
text in the section title.

BUG=None
TEST=mdbook build

Change-Id: I3dc4ae25e392b1260df59889926eba2044c4adc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3749605
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm