Fix minor mistake in ButtonWidget docs (#5050)

The ButtonWidget documentation refers to a "state" attribute, but this
should probably be "set". The name "state" appears to be an artifact
from when the setTitle, setField, and setIndex attributes were added to
the widget; at one time they were called stateTitle and so on, and then
they were renamed to setTitle instead, but not every instance of "state"
was found and renamed to "set". This is one such instance, now fixed.

editions/tw5.com/tiddlers/widgets/ButtonWidget.tid

@@ -29,7 +29,7 @@ The content of the `<$button>` widget is displayed within the button.
 |message |The name of the [[widget message|Messages]] to send when the button is clicked |
 |param |The optional parameter to the message |
 |set |A TextReference to which a new value will be assigned |
-|setTitle |A title to which a new value will be assigned, ''without'' TextReference. Gets preferred over <<.attr state>> |
+|setTitle |A title to which a new value will be assigned, ''without'' TextReference. Gets preferred over <<.attr set>> |
 |setField |A ''field name'' to which the new value will be assigned, if the attribute <<.attr setTitle>> is present. Defaults to the ''text'' field |
 |setIndex |An ''index'' to which the new value will be assigned, if the attribute <<.attr setTitle>> is present |
 |setTo |The new value to assign to the TextReference identified in the `set` attribute or the text field / the field specified through <<.attr setField>> / the index specified through <<.attr setIndex>> of the title given through <<.attr setTitle>> |