CHANGELOG: fix heading level for 1.9.1-rc1 #462

Job	Run time
generic-build-test (ubuntu-18.04, gcc)	7m 32s
generic-build-test (ubuntu-18.04, clang)	6m 35s
fuzzing-test	4m 30s
generic-build-test (ubuntu-20.04, gcc)	7m 13s
coverity-test	0s
generic-build-test (ubuntu-20.04, clang)	5m 45s
generic-build-test (fedora-30, gcc)	7m 17s
generic-build-test (fedora-30, clang)	6m 35s
generic-build-test (opensuse-leap, gcc)	7m 17s
coverage-test	7m 25s
generic-build-test (opensuse-leap, clang)	6m 12s
generic-build-test (ubuntu-22.04, gcc)	44m 50s
	1h 51m 11s

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