[DOC] Tweaks for StringIO#lineno= #550

Job	Run time
ruby-versions / Generate Ruby versions	3s
windows-latest 3.0	1m 0s
windows-latest 2.7	1m 31s
windows-latest 3.4	1m 5s
windows-latest mingw	52s
windows-latest 3.3	1m 2s
windows-latest 3.1	1m 7s
windows-latest mswin	1m 14s
windows-latest 3.2	1m 9s
windows-latest jruby-head	1m 40s
	10m 43s

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