pw_thread_zephyr: Fix doc headings

All top-level headings are included in the global ToC. Beause of this,
the "Thread Creation Backend" and "Thread Sleep Backend" headings from
pw_thread_zephyr were appearing in the global ToC.

By convention, the first heading should be H1 with the module name, and
all other headings should be H2+ and thus not included in the global
ToC.

Tested:
  $ gn gen out

  $ ninja -C out docs

  View out/docs/gen/docs/html/index.html and confirm that
  "Thread Creation Backend" and "Thread Sleep Backend" no longer
  appear in the global ToC.

Change-Id: I42cbacb90843a881e8b95ec016b84f9c67891f21
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/153191
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Jonathon Reinhart <jrreinhart@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
diff --git a/pw_thread_zephyr/docs.rst b/pw_thread_zephyr/docs.rst
index 4751fe5..535c1ed 100644
--- a/pw_thread_zephyr/docs.rst
+++ b/pw_thread_zephyr/docs.rst
@@ -1,8 +1,8 @@
 .. _module-pw_thread_zephyr:
 
-----------------
+================
 pw_thread_zephyr
-----------------
+================
 This is a set of backends for pw_thread based on the Zephyr RTOS.
 
 .. Warning::