doc: switch rst `=` section underlines to `-`

When using GENDOC_LOAD_CONFIGURED_EXTENSIONS=1, Mercurial's doc/gendoc.py can
load extensions from outside of just the hgext directory, and pull in this doc.

When added to the documentation, this should be nested inside of the following
extension hierarchy:

- Extensions
  - evolve

The generated documentation has an established hierarchy of section headers, and
the above uses `"` for top level, and `=` for each extension. Since gendoc just
pastes it in verbatim, with `=` underlines, we get "Obsolescence Markers
Discovery", "Effect Flag Experiment", and "Template keywords" as siblings to the
evolve extension, instead of nested inside of it.

This is not required for the string below it (that shows up with
`hg help evolution`), as that is one level higher in the hierarchy (not nested
inside of an "Extensions" section), so `=` is appropriate there.
8 jobs for topic/default/doc-tweaks in 4 minutes and 57 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed checks-py2 #168200

00:02:29

passed checks-py3 #168201

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passed doc #168206

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manual pytype #168199
allowed to fail manual
passed tests-py2-cext #168202

00:03:31

passed tests-py2-pure #168203

00:03:47

passed tests-py3-cext #168204

00:04:57

passed tests-py3-pure #168205

00:04:23