fix: allow interpolating names in @brownian

Author: AayushSabharwal

src/variables.jl

@@ -489,7 +489,9 @@ $(SIGNATURES)
 Define one or more Brownian variables.
 """
 macro brownian(xs...)
-    all(x -> x isa Symbol || Meta.isexpr(x, :call) && x.args[1] == :$, xs) ||
+    all(
+        x -> x isa Symbol || Meta.isexpr(x, :call) && x.args[1] == :$ || Meta.isexpr(x, :$),
+        xs) ||
         error("@brownian only takes scalar expressions!")
     Symbolics._parse_vars(:brownian,
         Real,