[DOC] Tweaks for StringIO.new #431

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 3.0	1m 49s
windows-latest 2.7	2m 4s
windows-latest 3.3	1m 44s
windows-latest 3.1	1m 41s
windows-latest 3.2	1m 58s
windows-latest mswin	1m 23s
windows-latest jruby-head	2m 23s
windows-latest mingw	1m 45s
windows-latest 3.4	1m 27s
	16m 18s

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