Fix spelling.

Author: andrew-thach

Lessons/04_operators.ipynb

@@ -154,7 +154,7 @@
    "source": [
     "## Order of Operations \n",
     "\n",
-    "Operations are applied in a specific order. Python uses the same order as many other programming languages. You can remember it using the word PEDMAS:\n",
+    "Operations are applied in a specific order. Python uses the same order as many other programming languages. You can remember it using the word PEMDAS:\n",
     "\n",
     "  * Parenthesis (this is done first)\n",
     "  * Exponents\n",