message: Fix JSON syntax in reference
diff: @@ -78,7 +78,8 @@ There are three kinds of instructions: constants, value operations, and effect o
  #### Constant
  - { "op": "const", "dest": "<string>", "type": <Type>, value: ... }
  + { "op": "const", "dest": "<string>", "type": <Type>,
  + "value": ... }
  A Constant is an instruction that produces a literal value.
  Its `op` field must be the string `"const"`.
  