Fix typo and reword comment about goog.object.setWithReturnValueIfNotSet
Fix typo and reword comment about goog.object.setWithReturnValueIfNotSet

Closes https://github.com/google/closure-library/pull/952.

RELNOTES: N/A

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=232065174

google/closure-library,google/closure-library,lucidsoftware/closure-library,lucidsoftware/closure-library,google/closure-library,google/closure-library,google/closure-library,lucidsoftware/closure-library,lucidsoftware/closure-library