Remove one JSON related MUST requirement

JSON is not a mandatory format, we should not use MUST
requirement on this.

Signed-off-by: Qiang Huang <[email protected]>

Author: hqhq

runtime.md

@@ -65,8 +65,6 @@ Unless otherwise stated, generating an error MUST leave the state of the environ
 
 This operation MUST generate an error if it is not provided the ID of a container.
 This operation MUST return the state of a container as specified in the [State](#state) section.
-In particular, the state MUST be serialized as JSON.
-
 
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