Update Exercises-1.ipynb (brandon-rhodes#30)

Fix typo

Author: Bonifacio2

Exercises-1.ipynb

@@ -919,7 +919,7 @@
       }
     }, 
     {
-      "source": "### How many roles were avilable for actresses in the 1950s?", 
+      "source": "### How many roles were available for actresses in the 1950s?", 
       "cell_type": "markdown", 
       "metadata": {
         "collapsed": true
@@ -1038,4 +1038,4 @@
       }
     }
   }
-}
+}