[DOC] Tweaks for StringIO#getc #559

Job	Run time
ruby-versions / Generate Ruby versions	5s
windows-latest 3.0	1m 3s
windows-latest 2.7	1m 30s
windows-latest 3.1	59s
windows-latest 3.2	1m 3s
windows-latest 3.3	56s
windows-latest mswin	58s
windows-latest 3.4	59s
windows-latest jruby-head	1m 42s
windows-latest mingw	52s
	10m 7s

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