Proof that a modified public-key encryption scheme is not CCA secure
So let's go through the CCA(2) game and see where things fall apart, shall we?


Challenger: Generates keys.
Adversary: Calls the encryption or decryption oracles a polynomial amount of times. We don't need this here.
Adversary: Picks two messages, eg $m_0=0$ and $m_1=1$.
Challenger: Chooses a bit $b$ uniformly at random, ie $b\in\{0,1\}$
Challenger: Encrypts $m_b$ and sends the result to the adversary as $C$, the challenge ciphertext.
Adversay: Perform a polynomial amount of operations including encryption or decryption oracle calls, e.g. to decrypt $C$ with the last bit flipped.
Adversary: Output a guess $b'$ for the bit $b$, eg the result of the decryption query in the last step.
If $b=b'$ the adversary wins. If the probability  of this happening is not negligbly higher than $1/2$, the scheme is broken.


Do you see now how CCA security is broken using 1 query?