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 <[e-mail]>
Reviewed-by: Randy Dunlap <[e-mail]>
Tested-by: Randy Dunlap <[e-mail]>
Reviewed-by: Jarkko Sakkinen <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

Documentation/security/keys/trusted-encrypted.rst

···
 =============================
 It is the secure way of keeping the keys in the kernel key-ring as Trusted-Key,
 such that:
+
 - Key-blob, an encrypted key-data, created to be stored, loaded and seen by
-            userspace.
+  userspace.
 - Key-data, the plain-key text in the system memory, to be used by
-            kernel space only.
+  kernel space only.
 
 Though key-data is not accessible to the user-space in plain-text, but it is in
 plain-text in system memory, when used in kernel space. Even though kernel-space