Fixup path to sphinx config file in readthedocs.yaml #123

Job	Run time
Build and test (ubuntu-22.04, 3.7)	40s
Build and test (ubuntu-22.04, 3.8)	36s
Build and test (ubuntu-22.04, 3.9)	44s
Build and test (ubuntu-22.04, 3.10)	33s
Build and test (ubuntu-22.04, 3.11)	38s
Build and test (ubuntu-22.04, 3.12)	41s
Build and test (ubuntu-22.04, pypy-3.7)	59s
Build and test (ubuntu-22.04, pypy-3.8)	57s
Build and test (ubuntu-22.04, pypy-3.9-nightly)	1m 14s
Build and test (ubuntu-22.04, pypy-3.10-nightly)	1m 14s
Build and test (macos-12, 3.7)	1m 15s
Build and test (macos-12, 3.8)	1m 36s
Build and test (macos-12, 3.9)	1m 12s
Build and test (macos-12, 3.10)	1m 28s
Build and test (macos-12, 3.11)	1m 54s
Build and test (macos-12, 3.12)	1m 28s
Build and test (macos-12, pypy-3.7)	1m 33s
Build and test (macos-12, pypy-3.8)	2m 56s
Build and test (macos-12, pypy-3.9-nightly)	2m 24s
Build and test (macos-12, pypy-3.10-nightly)	4m 42s
	28m 44s

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