[DOC] Tweaks for StringIO.open #433

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 2.7	2m 12s
windows-latest 3.2	1m 37s
windows-latest jruby-head	2m 26s
windows-latest 3.0	1m 32s
windows-latest 3.3	1m 47s
windows-latest 3.1	1m 42s
windows-latest 3.4	1m 34s
windows-latest mingw	1m 31s
windows-latest mswin	1m 21s
	15m 46s

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