Fix mistakes in MVar documentation #613

Job	Run time
Test without C extension	1m 20s
Test isolated (2.3)	1m 21s
Test isolated (ruby)	1m 21s
Tests: Ruby 2.3 - ubuntu	1m 20s
Tests: Ruby 2.4 - ubuntu	1m 20s
Tests: Ruby 3.1 - ubuntu	1m 21s
Tests: Ruby 3.0 - ubuntu	1m 22s
Tests: Ruby 2.7 - ubuntu	1m 21s
Tests: Ruby 2.5 - ubuntu	1m 21s
Tests: Ruby 3.4 - ubuntu	1m 21s
Tests: Ruby 2.6 - ubuntu	1m 21s
Tests: Ruby ruby - windows	1m 10s
Tests: Ruby jruby - ubuntu	1m 22s
Tests: Ruby 3.2 - ubuntu	1m 22s
Tests: Ruby truffleruby - ubuntu	1m 22s
Tests: Ruby 3.3 - ubuntu	1m 22s
	21m 27s

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