[DOC] Tweaks for StringIO#getc #547

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 2.7	1m 38s
windows-latest 3.4	1m 23s
windows-latest 3.2	1m 3s
windows-latest jruby-head	2m 2s
windows-latest 3.0	1m 1s
windows-latest 3.1	1m 12s
windows-latest mingw	53s
windows-latest mswin	1m 6s
windows-latest 3.3	1m 34s
	11m 56s

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