message: docs: add language tag to code blocks
diff: @@ -211,7 +211,7 @@ To avoid ugly intermediate variables, decaffeinate assumes that these operations
  may be safely repeated. For example, this code prints one of each statement in
  CoffeeScript and two of each statement after decaffeinate.
  -```
  +```coffee
  a = {valueOf: -> console.log 'Called valueOf'}
  Object.defineProperty(a, 'b', {
  get: ->
  @@ -230,7 +230,7 @@ identifier" test in the CoffeeScript test suite.
  For example, this code will fail after decaffeinate because `for own` results in
  code using `Object.keys`:
  -```
  +```coffee
  Object = 1
  for own k of {b: 1, c: 2}
  console.log k
  