Update 1_4_Sets.ipynb

Typo corrected.

python/1_4_Sets.ipynb

@@ -37,7 +37,7 @@
     "\n",
     "A set object is an unordered collection of distinct objects. There are two built-in set types: **set** (mutable) and **frozenset** (inmutable).\n",
     "\n",
-    "A mapping object maps hashable values to arbitrary objects. Mappings are mutable objects. There is only one bultin mapping type: **dictionary**."
+    "A mapping object maps hashable values to arbitrary objects. Mappings are mutable objects. There is only one builtin mapping type: **dictionary**."
    ]
   },
   {