Fix #7 - Histogram of distribution of error and warning types is wrong

[Suryakandukoori]

Summary:

Indexing of the errors in the histogram is off by one, it has now been corrected.

Expected behavior:

Histogram shows correct error frequency.

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• [X] Run the unit tests with mvn test to make sure you didn't break anything
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[barbeau]

Thanks @Suryakandukoori!