Fixed typo (aimacode#675)

Closes aimacode#673

Author: aswanipranjal

learning.ipynb

@@ -979,7 +979,7 @@
     "\n",
     "$$I_G(p) = \\sum{p_i(1 - p_i)} = 1 - \\sum{p_i^2}$$\n",
     "\n",
-    "We select split which minimizes the Gini impurity in childre nodes.\n",
+    "We select a split which minimizes the Gini impurity in child nodes.\n",
     "\n",
     "#### Information Gain\n",
     "Information gain is based on the concept of entropy from information theory. Entropy is defined as:\n",