update GTPSA docstring

Author: mattsignorelli

src/dense.jl

@@ -251,7 +251,7 @@ Defined by [ADTypes.jl](https://github.com/SciML/ADTypes.jl).
 
       + a GTPSA `Descriptor` specifying the number of variables/parameters, parameter
         order, individual variable/parameter truncation orders, and maximum order. See
-        the [GTPSA.jl documentation](https://bmad-sim.github.io/GTPSA.jl/stable/man/c_descriptor/) for more details.
+        the [GTPSA.jl documentation](https://bmad-sim.github.io/GTPSA.jl/stable/man/b_descriptor/) for more details.
       + `nothing` to automatically use a `Descriptor` given the context.
 """
 Base.@kwdef struct AutoGTPSA{D} <: AbstractADType