fix a crash error of "Attempt to access an null array"

[foreverlms]

Hi, I am new to Android and follow this practical to learn it. I when I have got to this practical, I find when I press the delete button of MainActivity an error happend at line 117 of WordListAdapter.java and the app crashed. I know the contentprovider will access the string array selectionArgs, but it seems to be null after initiaton. So I change the third positional argument of mContext.getContentResolver().delete() into new String[]{String.valueof(id)}. This works well. This is my first pull request. I hope I am not wrong.

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[foreverlms]

I signed it!

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CLAs look good, thanks!

[foreverlms]

Can I be reviewed?