document DHBackend is implemented for OpenSSL (#3304)

diff --git a/docs/hazmat/backends/openssl.rst b/docs/hazmat/backends/openssl.rst
index 6a5ae6f..f97ec57 100644
--- a/docs/hazmat/backends/openssl.rst
+++ b/docs/hazmat/backends/openssl.rst
@@ -15,6 +15,7 @@
     * :class:`~cryptography.hazmat.backends.interfaces.CipherBackend`
     * :class:`~cryptography.hazmat.backends.interfaces.CMACBackend`
     * :class:`~cryptography.hazmat.backends.interfaces.DERSerializationBackend`
+    * :class:`~cryptography.hazmat.backends.interfaces.DHBackend`
     * :class:`~cryptography.hazmat.backends.interfaces.DSABackend`
     * :class:`~cryptography.hazmat.backends.interfaces.EllipticCurveBackend`
     * :class:`~cryptography.hazmat.backends.interfaces.HashBackend`