Fix contributing md link in building.md #1752

Job	Run time
Run checks	1m 0s
Run benchmarks	1m 33s
Run tests (emulated riscv64)	1m 33s
Check relative links in website	31s
Run tests (ubuntu, 1.23.x)	1m 41s
Run tests (emulated s390x)	1m 37s
Run tests (macos, 1.23.x)	1m 56s
Run tests (windows, 1.23.x)	2m 42s
Run tests (ubuntu, 1.22.x)	1m 20s
Upload test coverage (linux)	14s
Upload test coverage (macos)	16s
Upload test coverage (windows)	22s
	14m 45s

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