Format code block in header

Adding some triple tildes makes doxygen format code blocks
correctly.

Merged from https://widevine-internal-review.googlesource.com/178230

Change-Id: Ie4cce625fbd5f77a2f65191716bd4a2a8cf8b76e

libwvdrmengine/oemcrypto/include/OEMCryptoCENC.h

@@ -4873,6 +4873,7 @@ OEMCryptoResult OEMCrypto_GetBootCertificateChain(
  * 4.4, as well as Android IRemotelyProvisionedComponent.aidl (under
  * "SignedData<Data>")
  *
+ * ~~~
  *   |public_key_signature|: COSE_Sign1 CBOR array
  *   [
  *     protected: bstr .cbor { 1 : AlgorithmEdDSA / AlgorithmES256 /
@@ -4882,6 +4883,7 @@ OEMCryptoResult OEMCrypto_GetBootCertificateChain(
  *     signature: bstr      ; PureEd25519(priv_key, Sig_structure) /
  *                          ; ECDSA(priv_key, Sig_structure)
  *   ]
+ * ~~~
  *
  * Notes:
  *   1. The payload field in the COSE_Sign1 struct is the public key generated
@@ -4896,6 +4898,7 @@ OEMCryptoResult OEMCrypto_GetBootCertificateChain(
  *      field in Sig_structure is the same as the payload in the above
  *      COSE_Sign1 CBOR array.
  *
+ * ~~~
  *   Sig_structure: CBOR array
  *   [
  *     context: "Signature1",
@@ -4904,6 +4907,7 @@ OEMCryptoResult OEMCrypto_GetBootCertificateChain(
  *     external_aad: bstr .size 0,
  *     payload: bstr .cbor Data / nil,
  *   ]
+ * ~~~
  *
  * If an OEM private key is available, the request is assumed to be for DRM
  * certificate provisioning and the public key is signed by the OEM private key.