config: Fix 'optional' -> 'OPTIONAL' for 'windows'

The shift happened in c35cf57 (config: Replace "optional" with
"OPTIONAL", 2016-09-17, opencontainers#574) and the 'windows' entry landed in
parallel with dc8f2c2 (Add support for Windows-based containers,
2016-09-16, opencontainers#573).

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

Author: wking

config.md

@@ -292,7 +292,7 @@ For Windows based systems the user structure has the following fields:
   This SHOULD only be set if **`platform.os`** is `linux`.
 * **`solaris`** (object, OPTIONAL) [Solaris-specific configuration](config-solaris.md).
   This SHOULD only be set if **`platform.os`** is `solaris`.
-* **`windows`** (object, optional) [Windows-specific configuration](config-windows.md).
+* **`windows`** (object, OPTIONAL) [Windows-specific configuration](config-windows.md).
   This SHOULD only be set if **`platform.os`** is `windows`.
 
 ### Example (Linux)