[DOC] Tweaks for StringIO#string= #499

Job	Run time
ruby-versions / Generate Ruby versions	3s
windows-latest 3.0	1m 5s
windows-latest 3.3	1m 2s
windows-latest 3.4	54s
windows-latest 3.1	1m 28s
windows-latest mingw	1m 1s
windows-latest jruby-head	1m 57s
windows-latest 3.2	1m 38s
windows-latest 2.7	1m 51s
windows-latest mswin	1m 13s
	12m 12s

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