libtsxx [FIX][DOC]: use correct index (was a typo)

src/libs/libtsxx/correlate.h

@@ -53,7 +53,7 @@ namespace ts {
    * \f$ b_l: l \in [F_b,L_b]. \f$
    * From the second factor we conclude
    * \f$ F_b -l \leq k \leq L_b - l \f$ and thus the index range of 
-   * \f$ c_l \f$ is
+   * \f$ c_k \f$ is
    * \f[
    *    F_b - L_a \leq k \leq L_b - F_a.
    * \f]