Fix second example in README.md

I'm not very familiar with either `Julia` or `ModelingToolkit`, but copy pasting the example as it currently stands leads to the following error:

```
ArgumentError: Variable a is not a function of independent variable t.
```

Making the above change seems to resolve it.

Author: mayl

README.md

@@ -77,7 +77,7 @@ eqs = [D(x) ~ σ*(y-x),
 lorenz1 = ODESystem(eqs,name=:lorenz1)
 lorenz2 = ODESystem(eqs,name=:lorenz2)
 
-@variables a
+@variables a(t)
 @parameters γ
 connections = [0 ~ lorenz1.x + lorenz2.y + a*γ]
 connected = ODESystem(connections,t,[a],[γ],systems=[lorenz1,lorenz2])