Fixed grammar in CheckReturnValue.md: "Code that doing exception-testing" -> "Code that does exception testing".
Fixed grammar in CheckReturnValue.md: "Code that doing exception-testing" -> "Code that does exception testing".

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=202725410

google/error-prone,cushon/error-prone,cushon/error-prone,google/error-prone,cushon/error-prone,cushon/error-prone