fix typo in a tutorial (#52)

Author: wojdyr

docs/sources/tutorials/Working_with_PDB_Structures_in_DataFrames.md

@@ -218,7 +218,7 @@ ppdb.df.keys()
 [File link: [3eiy.pdb](https://raw.githubusercontent.com/rasbt/biopandas/master/docs/sources/tutorials/data/3eiy.pdb)]
 
 - 'ATOM': contains the entries from the ATOM coordinate section
-- 'ATOM':  ... entries from the "HETATM" coordinate section    
+- 'HETATM':  ... entries from the "HETATM" coordinate section    
 - 'ANISOU': ... entries from the "ANISOU" coordinate section 
 - 'OTHERS': Everything else that is *not* a 'ATOM', 'HETATM', or 'ANISOU' entry