fix typo in linear recurrence description

algorithms/math/linear-recurrence.tex

@@ -1,5 +1,5 @@
 \subsection{Linear Recurrence}
 
-Find the n-th term of a linear recurrence, given the recurrence $rec$ and the first $K$ values of the recurrence, remember that first\_k[i] if the value of $f(i)$, considering 0-indexing. 
+Find the n-th term of a linear recurrence, given the recurrence $rec$ and the first $K$ values of the recurrence, remember that first\_k[i] is the value of $f(i)$, considering 0-indexing. 
 
 time: $O(K^{3} \log{N})$