[DOC] Tweaks for StringIO#getc (#189) #560

Job	Run time
ruby-versions / Generate Ruby versions	3s
windows-latest 2.7	1m 39s
windows-latest 3.0	1m 8s
windows-latest 3.2	56s
windows-latest 3.3	1m 9s
windows-latest mingw	1m 5s
windows-latest 3.4	1m 9s
windows-latest 3.1	58s
windows-latest jruby-head	1m 33s
windows-latest mswin	1m 30s
	11m 10s

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