docs(equations): added missing \f for latex rendering

src/include/serif/equations/mass_normalization.hpp

@@ -98,7 +98,7 @@ namespace serif::equations {
          *
          * @note The backend actually adds whatever is returned from this function to the integrated residual to get the final residual. Therefore we return the negative of the target mass here.
          *
-         * @return The negative of the target mass, $-M_{\text{target}}$.
+         * @return The negative of the target mass, \f$-M_{\text{target}}\f$.
          */
         [[nodiscard]] double residual_offset() const {
             return -m_target_mass;