[DOC] Fix links in StringIO #583

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 3.1	1m 33s
windows-latest 3.2	2m 4s
windows-latest 3.3	1m 20s
windows-latest 3.0	1m 55s
windows-latest 2.7	2m 11s
windows-latest mswin	1m 13s
windows-latest 4.0	1m 24s
windows-latest 3.4	1m 39s
windows-latest mingw	1m 27s
windows-latest jruby-head	2m 9s
	16m 59s

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