CHANGELOG: fix heading level for 1.9.1-rc1 #463

Job	Run time
generic-build-test (ubuntu-18.04, gcc)	7m 6s
generic-build-test (ubuntu-18.04, clang)	6m 13s
coverage-test	7m 9s
generic-build-test (ubuntu-20.04, gcc)	6m 53s
generic-build-test (ubuntu-20.04, clang)	5m 48s
generic-build-test (fedora-30, gcc)	7m 18s
fuzzing-test	4m 33s
generic-build-test (fedora-30, clang)	6m 42s
generic-build-test (opensuse-leap, gcc)	7m 13s
generic-build-test (opensuse-leap, clang)	6m 10s
generic-build-test (ubuntu-22.04, gcc)	44m 36s
coverity-test	0s
	1h 49m 41s

Provide feedback