Fix "type object 'DataType' has no attribute 'DataTypeString'" error

[jdugge]

When changing the plot type to "pie", I get the following error message:

 'color', QgsPropertyDefinition.DataType.DataTypeString, 'Color Array',
AttributeError: type object 'DataType' has no attribute 'DataTypeString'

The effect of this error is that the "format help" in the expression editor for the marker color is not updated correctly.

This error is caused by an incorrect call to the DataTypeString constant, which is fixed in this PR