Apply suggestion from @kellertuer

Author: kellertuer

src/solvers/proximal_gradient_method.jl

@@ -46,7 +46,7 @@ $(_kwargs(:stepsize; default = "`[`default_stepsize`](@ref)`(M, `[`ProximalGradi
   that by default uses a [`ProximalGradientMethodBacktracking`](@ref).
 $(_kwargs(:retraction_method))
 $(_kwargs(:stopping_criterion; default = "`[`StopAfterIteration`](@ref)`(100)"))
-$(_kwargs(:sub_problem; name = "sub_problem", type = "Union{AbstractManoptProblem, F, Nothing}", default = "nothing"))
+$(_kwargs(:sub_problem; type = "Union{AbstractManoptProblem, F, Nothing}", default = "nothing"))
   or nothing to take the proximal map from the [`ManifoldProximalGradientObjective`](@ref)
 $(_kwargs(:sub_state; default = "evaluation")). This field is ignored, if the `sub_problem` is `Nothing`.