DOC: Fix docstrings for boxplot, get_group, and indices methods
Fixes docstrings of below methods

```
-i "pandas.core.groupby.DataFrameGroupBy.boxplot PR07,RT03,SA01" \ 
 -i "pandas.core.groupby.DataFrameGroupBy.get_group RT03,SA01" \ 
 -i "pandas.core.groupby.DataFrameGroupBy.indices SA01" \ 
```
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