message: Add "then" keyword to the docs
diff: @@ -79,6 +79,7 @@ error: found a floating point literal, but expected a type `U64`
  | `let` | [bindings] |
  | `record` | [record] values |
  | `Record` | [Record] types |
  +| `then` | [if-then-else-expressions] |
  | `Type` | [polymorphic functions], [types of types] |
  | `where` | |
  