[docs] fix link to HiGHS GPU

Author: odow

docs/src/parallel.md

@@ -7,7 +7,7 @@ computing. When using a CPU, these are currently restricted to the
 dual simplex solver for LP, the factorisation-based interior point solver,
 and the MIP solver. Details of these and future plans are set out below. 
 HiGHS has an implementation of a first order method (PDLP) for solving LPs 
-that can exploit the availability of a [GPU](@ref highs-gpu).
+that can exploit the availability of a [GPU](@ref highs).
 
 By default, when running in parallel, HiGHS will use half the
 available threads on a machine. This number can be modified by setting