Update doc/source/groupby.rst

Fixed a small typo in the docs
Merge pull request #2581 from parlarjb/patch-1

Update doc/source/groupby.rst
BUG: except raised if plot parameters has both color and style with color #2588
BUG: label setting failure in xy plot #2543
BUG: groupby agg over PeriodIndex unboxes to ordinals #2569
BUG: time converter bug computing the hour
BUG: DatetimeEngine does not handle pydatetime with tz #2575
BUG: Series arithmetic operator cannot handle constant and ndarray #2574
BUG: hierarchical indexing bug if more than 2 levels and top is empty #2596
ENH: add cleaned up pypi stats aggregation
RLS: bump to 0.10.1dev
BUG: fix groupby regression introduced in 0.10 breaking book example. close #2605
Update docstring to reflect deprecation of force_unicode
Add parameter deprecation warnings
ENH: return DatetimeIndex with metadata from unique. close #2563
ENH: better impl for na_map in string methods. close #2602
BUG: fix integer overflow possibility in python parser. close #2601
ENH/BUG/DOC: added support for data column queries (can construct searches on the actual columns of the data)
             added nan_rep for supporting string columns with nan's in them
             performance enhancements on string columns
             more tests & docs for data columns
removed conf.py paths
BUG: support multiple data columns that are in the same block (e.g. the same type)
     e.g. self.store.select('df', [ Term('string', '=', 'foo'), Term('string2=foo'), Term('A>0'), Term('B<0') ])