Add a filename to anchor links in build docs.

The href base added by I8a8ac0f9ba6c0d57ed83db2155955a95e070a265
doesn't really play nice with anchor links. This fixes the issue.

Fixes: 126550408
Test: m soong_docs
Change-Id: If62411a6391fd8151fa40cb2ecf32c5006797d6c