Use keyword arguments in mappable specs

We were getting a warning with Ruby 2.7 due to the change in the way
keyword arguments are handled in Ruby 3.0.

```
ruby/gems/2.7.0/gems/rspec-support-3.11.0/lib/rspec/support/with_keywords_when_needed.rb:18:
warning: Passing the keyword argument as the last hash parameter is
deprecated
```

As hinted by the warning, this code crashes with Ruby 3.0:

```
ArgumentError:
  unknown keyword: :budget_id
```

I'm not sure why this is the case, though, since we were already
explicitely passing a hash first before passing the keyword parameters.
I guess there are some cases in this whole keyword VS hash parameters
incompatibility that I haven't completely understood.

spec/system/budgets/investments_spec.rb

@@ -1130,7 +1130,7 @@ describe "Budget Investments" do
                   "new_budget_investment_path",
                   "",
                   "budget_investment_path",
-                  { budget_id: "budget_id" }
+                  mappable_path_arguments: { budget_id: "budget_id" }
 
   context "Destroy" do
     scenario "Admin cannot destroy budget investments", :admin do