feat(handouts): Clarify geo series in Summation

handouts/Summation/Summation.tex

@@ -868,6 +868,7 @@ \subsection{Linear recurrences}
   \[
     L(x) = \sum_{n \ge 0} L_n x^n
     = \frac{1}{1-\alpha x} + \frac{1}{1 - \beta x}
+    = \sum_{n \ge 0} (\alpha x)^n + \sum_{n \ge 0} (\beta x)^n
     = \sum_{n \ge 0} \left( \alpha^n + \beta^n \right)x^n.
   \]
   So we're done upon equating coefficients.