Fix HighlightTrigger.background_color property getter signature (gnachman#592)

The @Property getter had a stray `value` parameter copied from the
setter, causing TypeError when accessing trigger.background_color.

Author: mkusaka

api/library/python/iterm2/iterm2/triggers.py

@@ -711,7 +711,7 @@ def text_color(self, value: typing.Optional[iterm2.Color]):
         self.param = self._param
 
     @property
-    def background_color(self, value: typing.Optional[iterm2.Color]):
+    def background_color(self) -> typing.Optional[iterm2.Color]:
         return self.__background_color
 
     @background_color.setter