Documentation: crypto: crypto_engine: Fix Sphinx warning

This fixes the following Sphinx warning:

Documentation/crypto/crypto_engine.rst:2:
  WARNING: Explicit markup ends without a blank line; unexpected unindent.
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/crypto/crypto_engine.rst

 .. SPDX-License-Identifier: GPL-2.0
+
 Crypto Engine
 =============