config: Fix 'optional' -> 'OPTIONAL' for process.terminal

This happened in c35cf57 (config: Replace "optional" with "OPTIONAL",
2016-09-17, opencontainers#574) but was accidentally rolled back in 52f3cde
(Clarify wording for terminal setting and /dev/console, 2016-07-18,
opencontainers#518).

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

config.md

@@ -118,7 +118,7 @@ See links for details about [mountvol](http://ss64.com/nt/mountvol.html) and [Se
 
 **`process`** (object, REQUIRED) configures the container process.
 
-* **`terminal`** (bool, optional) specifies whether a terminal is attached to that process, defaults to false.
+* **`terminal`** (bool, OPTIONAL) specifies whether a terminal is attached to that process, defaults to false.
   On Linux, a pseudoterminal pair is allocated for the container process and the pseudoterminal slave is duplicated on the container process's [standard streams][stdin.3].
 * **`consoleSize`** (object, OPTIONAL) specifies the console size of the terminal if attached, containing the following properties:
   * **`height`** (uint, REQUIRED)