Fix typo in Promise docs #579

Job	Run time
Test without C extension	3m 6s
Test isolated (2.3)	3m 38s
Tests: Ruby 2.3 - ubuntu	3m 24s
Test isolated (ruby)	3m 23s
Tests: Ruby 2.4 - ubuntu	3m 17s
Tests: Ruby 2.5 - ubuntu	3m 18s
Tests: Ruby 2.6 - ubuntu	3m 22s
Tests: Ruby 2.7 - ubuntu	3m 16s
Tests: Ruby 3.0 - ubuntu	3m 3s
Tests: Ruby 3.1 - ubuntu	3m 9s
Tests: Ruby 3.2 - ubuntu	3m 7s
Tests: Ruby 3.3 - ubuntu	3m 10s
Tests: Ruby 3.4 - ubuntu	3m 4s
Tests: Ruby jruby - ubuntu	5m 40s
Tests: Ruby truffleruby - ubuntu	4m 9s
Tests: Ruby ruby - windows	5m 11s
	57m 17s

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