fix: fix deprecated @brownian not working properly

Author: AayushSabharwal

src/deprecations.jl

@@ -175,5 +175,5 @@ macro brownian(xs...)
     return quote
         Base.depwarn("`@brownian` is deprecated. Use `@brownians` instead", :brownian_macro)
         $(@__MODULE__).@brownians $(xs...)
-    end
+    end |> esc
 end

test/sdesystem.jl

@@ -948,4 +948,8 @@ end
 
 @testset "`@brownian` is deprecated" begin
     @test_deprecated @brownian a b c
+
+    @brownian p q
+    @test ModelingToolkit.isbrownian(p)
+    @test ModelingToolkit.isbrownian(q)
 end