incorrect @return docs for ElementInfoManagerInterface::getInfo()

Problem/Motivation

The docs say:

   * @return array
   *   An associative array describing the element types being defined. The
   *   array contains a sub-array for each element type, with the
   *   machine-readable type name as the key

but this is wrong, as the method is for getting the info for a *single* type, as the first line describes:

> * Retrieves the default properties for the defined element type.

There is no sub-array, and there is only one element type.

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