Fix some minor errors in the Groovy README
Fix some minor errors in the Groovy README

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Change-Id: Ibe3978cd85749f9dadbdfc6e0469a25325bd1b69
Reviewed-on: https://bazel-review.googlesource.com/#/c/2263/
MOS_MIGRATED_REVID=107689272

robbiemc/rules_groovy,bazelbuild/rules_groovy