Fix heading levels in README and Changelog #315

Job	Run time
coverage (2.7, ubuntu-22.04)	34s
profile (2.7, native, ubuntu-22.04)	2m 16s
profile (2.7, pure, ubuntu-22.04)	2m 17s
test (2.3, ubuntu-22.04, Gemfile)	44s
test (2.4, ubuntu-22.04, Gemfile)	45s
test (2.5, ubuntu-22.04, Gemfile)	46s
test (2.6, ubuntu-22.04, Gemfile)	37s
test (2.7, ubuntu-22.04, Gemfile)	34s
test (3.0, ubuntu-22.04, Gemfile)	35s
test (3.1, ubuntu-22.04, Gemfile)	35s
test (3.2, ubuntu-22.04, Gemfile)	39s
test (3.3, ubuntu-22.04, Gemfile)	40s
test (jruby-9.1, ubuntu-22.04, Gemfile)	50s
test (jruby-9.2, ubuntu-22.04, Gemfile)	1m 5s
test (jruby-9.3, ubuntu-22.04, Gemfile)	1m 4s
test (jruby-9.4, ubuntu-22.04, Gemfile)	57s
test (truffleruby-22.2, ubuntu-22.04, Gemfile)	2m 27s
test (truffleruby-22.3, ubuntu-22.04, Gemfile)	2m 30s
test (ubuntu-22.04, 2.7, gemfiles/public_suffix_2.rb)	1m 5s
test (ubuntu-22.04, 2.7, gemfiles/public_suffix_3.rb)	1m 10s
test (ubuntu-22.04, 2.7, gemfiles/public_suffix_4.rb)	42s
test (macos-13, 3.1)	34s
test (windows-2019, 3.1)	2m 17s
test (windows-2022, 3.1)	1m 50s
test (windows-2022, jruby-9.3)	1m 4s
test (ubuntu-22.04, head, Gemfile, true)	26s
test (ubuntu-22.04, jruby-head, Gemfile, true)	59s
test (ubuntu-22.04, truffleruby-head, Gemfile, true)	1m 11s
	31m 13s

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