[ComplexArithmetic] Fix typo in the workbook (microsoft#795)

tutorials/ComplexArithmetic/Workbook_ComplexArithmetic.ipynb

@@ -648,7 +648,7 @@
     ">\n",
     "> We can simplify this using the following trigonometric identities:\n",
     "> * $\\cos a \\cos b  \\mp \\sin a \\sin b = \\cos(a \\pm b)$\n",
-    "> * $\\sin a \\cos b  \\pm \\sin a \\cos b = \\sin(a \\pm b)$\n",
+    "> * $\\sin a \\cos b  \\pm \\sin b \\cos a = \\sin(a \\pm b)$\n",
     ">\n",
     "> Finally, this solution gives the same answer as the short solution above:\n",
     ">$$z = r_{1}r_{2}(\\cos(\\theta_1 + \\theta_2) + i \\sin(\\theta_1 + \\theta_2)) = r_{1}r_{2} \\cdot e^{(\\theta_1 + \\theta_2)i} $$"