config: Remove redundant OPTIONAL from Labels description

While the other annotations references (e.g. for manifest annotations)
use the "This OPTIONAL property..." pattern, config.md puts the
OPTIONAL/REQUIRED-ness in the same line as the property name (while
manifest.md does not).  My preferred larger fix would be to follow
runtime-spec and consistently declare OPTIONAL/REQUIRED-ness in the
same line as the property name, but that's a bigger change.  This
commit just removes the redundant OPTIONAL from the description body.

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

Author: wking

config.md

@@ -160,7 +160,7 @@ Note: Any OPTIONAL field MAY also be set to null, which is equivalent to being a
    - **Labels** *object*, OPTIONAL
 
      The field contains arbitrary metadata for the container.
-     This OPTIONAL property MUST use the [annotation rules](annotations.md#rules).
+     This property MUST use the [annotation rules](annotations.md#rules).
 
   - **StopSignal** *string*, OPTIONAL