[DOC] Tweaks for StringIO#getc #618

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.2	19s
ubuntu-24.04 2.7	45s
ubuntu-24.04 3.1	15s
ubuntu-24.04 3.0	11s
ubuntu-24.04 3.4	18s
ubuntu-24.04 head	13s
ubuntu-24.04 3.3	18s
ubuntu-24.04 truffleruby-head	31s
ubuntu-24.04 debug	23s
ubuntu-22.04 3.2	23s
ubuntu-22.04 3.0	16s
ubuntu-24.04 jruby-head	1m 7s
ubuntu-22.04 3.1	14s
ubuntu-22.04 head	13s
ubuntu-22.04 truffleruby-head	25s
ubuntu-22.04 2.7	44s
ubuntu-22.04 jruby-head	1m 8s
ubuntu-22.04 3.3	20s
ubuntu-22.04 debug	24s
ubuntu-22.04 3.4	19s
	8m 50s

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