Typo fix: underscoes => underscores #1921

Job	Run time
RuboCop	31s
specs (ubuntu, 3.1.6)	1m 35s
specs (ubuntu, 3.2.7)	1m 33s
specs (ubuntu, 3.3.7)	1m 25s
specs (ubuntu, 3.4.2)	1m 32s
specs (macos, 3.1.6)	1m 40s
specs (macos, 3.2.7)	1m 16s
specs (macos, 3.3.7)	1m 29s
specs (macos, 3.4.2)	1m 26s
specs (windows, 3.1.6)	4m 20s
specs (windows, 3.2.7)	3m 45s
specs (windows, 3.3.7)	4m 3s
specs (windows, 3.4.2)	4m 41s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 24s
	30m 40s

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