Finding the transfer function of this RC circuit
In order for the \$R\$ and \$C\$ to be in parallel, you would need \$V_{\text{out}} = 0\$ due to a short circuit. But that's not the case.

First calculate \$V_{\text{out+}}\$, the voltage at the + terminal of \$V_{\text{out}}\$. This is just a voltage divider:

$$V_{\text{out+}} = \frac{1/sC}{1/sC + R}V_{\text{in}} = \frac{1}{1 + sRC}V_{\text{in}}$$

Now calculate \$V_{\text{out-}}\$, the voltage at the - terminal of \$V_{\text{out}}\$. This is also just a voltage divider:

$$V_{\text{out-}} = \frac{R}{1/sC + R}V_{\text{in}} = \frac{sRC}{1 + sRC}V_{\text{in}}$$

Now you can calculate \$V_{\text{out}}\$:

$$V_{\text{out}} = V_{\text{out+}} - V_{\text{out-}}$$