docs: trusted-encrypted: fix htmldocs build error

docs: trusted-encrypted: fix htmldocs build error

Running "make htmldocs" generates the following build error and
warning in trusted-encrypted.rst:

Documentation/security/keys/trusted-encrypted.rst:18: ERROR: Unexpected indentation.
Documentation/security/keys/trusted-encrypted.rst:19: WARNING: Block quote ends without a blank line; unexpected unindent.

Add a blank line before bullet list and fix the indentation of text to
fix the build error and resolve the warning.

Fixes: 38f6880759fd ("docs: trusted-encrypted: trusted-keys as protected keys")
Signed-off-by: Gopi Krishna Menon <krishnagopi487@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>