Merge pull request jakevdp#35 from pkitslaar/patch-1

Fixed typo

Author: jakevdp

notebooks/03.08-Aggregation-and-Grouping.md

@@ -74,7 +74,7 @@ This has some details on the 1,000+ extrasolar planets discovered up to 2014.
 ## Simple Aggregation in Pandas
 
 
-Earler, we explored some of the data aggregations available for NumPy arrays (["Aggregations: Min, Max, and Everything In Between"](02.04-Computation-on-arrays-aggregates.ipynb)).
+Earlier, we explored some of the data aggregations available for NumPy arrays (["Aggregations: Min, Max, and Everything In Between"](02.04-Computation-on-arrays-aggregates.ipynb)).
 As with a one-dimensional NumPy array, for a Pandas ``Series`` the aggregates return a single value:
 
 ```python