Bump rexml from 3.3.6 to 3.3.9 in /docs #1866

Job	Run time
build-macos-brew (clang-9)	3m 23s
build-linux (gcc-9)	5m 17s
build-linux-Qt6 (gcc-9)	6m 20s
build-macos-brew-Qt6 (clang-9)	4m 10s
build-macos-no-brew (clang-9)	7m 3s
build-macos-no-brew-Qt6 (clang-9)	8m 31s
build-windows-mingw (5.15.*)	14m 47s
build-windows-mingw-Qt6 (6.5.*, qtmultimedia qt5compat)	18m 33s
build-windows-vs (5.15.*)	12m 13s
build-windows-vs-Qt6 (6.3.*, qtmultimedia qt5compat)	19m 3s
	1h 39m 20s

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