[CSS] Fix <0x01> in completion labels (#2454)

Looks like python requires a raw string to perform proper regexp
replacement. The intention here is to replace `${1:param}` by `param` so
the completion list displays named function arguments.

CSS/css_completions.py

@@ -639,7 +639,7 @@ def complete_property_value(self, view, prefix, pt):
 
                 for value in values:
                     if isinstance(value, str):
-                        desc = self.re_trigger.sub("\1", value)
+                        desc = self.re_trigger.sub(r"\1", value)
                         snippet = value
                     else:
                         desc, snippet = value