message: Fix typo in single_include, too
diff: @@ -13046,7 +13046,7 @@ class basic_json
  @return copy of the element at key @a key or @a default_value if @a key
  is not found
  - @throw type_error.306 if the JSON value is not an objec; in that case,
  + @throw type_error.306 if the JSON value is not an object; in that case,
  using `value()` with a key makes no sense.
  @complexity Logarithmic in the size of the container.
  