Fix grammatical error in introduction chapter

paper/chapters/intro.tex

@@ -436,7 +436,7 @@ \section{Sums of Tensors}
 Even something as simple as a sum of two vectors, $x+y$, can not be displayed with a simple contraction graph.
 (Note that this is not linear because $ax+by\neq ab(x+y)$.)
 
-To handle this important operation, Penrose suggesting simply writing the two graphs with a plus sign between them, such as $-x + -y$.
+To handle this important operation, Penrose suggested simply writing the two graphs with a plus sign between them, such as $-x + -y$.
 Note that this is itself an order-1 tensor, even though it may look like there are two free edges.
 If we want to multiply the sum with another tensor, we can use parentheses like $-M\!-\!(-x + -y)$.