Fix typo in get_unit for ops

I'm almost sure this is a typo since otherwise the try/catch block
doesn't makes sense and fixing this fixes an error during documentation
build.

Author: contradict

src/systems/unit_check.jl

@@ -78,7 +78,7 @@ get_unit(op::typeof(instream), args) = get_unit(args[1])
 function get_unit(op, args) # Fallback
     result = op(get_unit.(args)...)
     try
-        result
+        get_unit(result)
     catch
         throw(ValidationError("Unable to get unit for operation $op with arguments $args."))
     end

test/units.jl

@@ -220,3 +220,6 @@ end
 
 @named sys = ArrayParamTest(a = [1.0, 3.0]u"cm")
 @test ModelingToolkit.getdefault(sys.a) ≈ [0.01, 0.03]
+
+@variables x(t)
+@test ModelingToolkit.get_unit(sin(x)) == ModelingToolkit.unitless