Fixup internal links in `CONTRIBUTING.md` #6264

Job	Run time
ios-build	6m 40s
web/samples_index config check	44s
Test Flutter stable on ubuntu-latest	17m 5s
Test Flutter stable on macos-latest	16m 23s
Test Flutter stable on windows-latest	26m 8s
Test Flutter beta on ubuntu-latest	15m 25s
Test Flutter beta on macos-latest	15m 26s
Test Flutter beta on windows-latest	24m 53s
Test Flutter master on ubuntu-latest	15m 52s
Test Flutter master on macos-latest	13m 38s
Test Flutter master on windows-latest	24m 7s
	2h 56m 21s

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