Fixes broken links in the table of contents of README #1299

Job	Run time
test-memory (head)	26s
check-misc (head)	6s
test-windows (head)	2m 55s
steep-check (head)	15s
test-ruby (head, master)	5m 58s
test-ruby (3.0, master)	5m 37s
ruby-versions / Generate Ruby versions	3s
test (2.5)	22s
test (2.6)	24s
test (2.7)	23s
test (3.0)	11s
test (3.1)	10s
test (3.2)	15s
test (3.3)	11s
test (head)	11s
	17m 27s

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