updated introduction

nb/0_introduction.ipynb

@@ -19,10 +19,10 @@
     "**Got Pandas? _Practical Data Wrangling with Pandas_**\n",
     "\n",
     "* **Introduction**\n",
-    "1. [Data Structures](./0_data_structures.ipynb)\n",
-    "2. [Importing Data](./1_importing_data.ipynb)\n",
-    "3. [Manipulating DataFrames](./2_dataframe_operations.ipynb)\n",
-    "4. [Wrap Up](3_wrapping_up.ipynb)\n",
+    "1. [Data Structures](./1_data_structures.ipynb)\n",
+    "2. [Importing Data](./2_importing_data.ipynb)\n",
+    "3. [Manipulating DataFrames](./3_dataframe_operations.ipynb)\n",
+    "4. [Wrap Up](./4_wrapping_up.ipynb)\n",
     "\n",
     "---"
    ]