Fix typo

chapter26.tex

@@ -73,7 +73,7 @@ For example, the subsequences of
 \index{prefix}
 \index{suffix}
 
-A \key{prefix} is a subtring that starts at the beginning
+A \key{prefix} is a substring that starts at the beginning
 of a string,
 and a \key{suffix} is a substring that ends at the end
 of a string.