Update index.md

contrib/pandas/index.md

@@ -1,10 +1,10 @@
 # List of sections
 
 - [Pandas Introduction and Dataframes in Pandas](introduction.md)
-- [Pandas Series Vs NumPy ndarray](pandas_series_vs_numpy_ndarray.md)
-- [Pandas Descriptive Statistics](Descriptive_Statistics.md)
-- [Group By Functions with Pandas](GroupBy_Functions_Pandas.md)
-- [Excel using Pandas DataFrame](excel_with_pandas.md)
+- [Pandas Series Vs NumPy ndarray](pandas-series-vs-numpy-ndarray.md)
+- [Pandas Descriptive Statistics](descriptive-statistics.md)
+- [Group By Functions with Pandas](groupby-functions.md)
+- [Excel using Pandas DataFrame](excel-with-pandas.md)
 - [Working with Date & Time in Pandas](datetime.md)
 - [Importing and Exporting Data in Pandas](import-export.md)
 - [Handling Missing Values in Pandas](handling-missing-values.md)