Typo

Advanced/De Bruijn/parts/0 intro.tex

@@ -9,7 +9,7 @@ once the sequence \texttt{010} is entered.
 \vspace{2mm}
 
 Naturally, there are $2^2 = 4$ possible combinations that open this lock. \par
-If don't know the lock's combination, we could try to guess it by trying all four combinations. \par
+If we don't know the lock's combination, we could try to guess it by trying all four combinations. \par
 This would require eight key presses: \texttt{0001101100}.
 
 \problem{}