Tiny docstring clean-up

httpie/cli/argtypes.py

@@ -62,7 +62,7 @@ def __init__(self, *separators: str):
             self.special_characters.update(separator)
 
     def __call__(self, s: str) -> KeyValueArg:
-        """Parse raw string arg  and return `self.key_value_class` instance.
+        """Parse raw string arg and return `self.key_value_class` instance.
 
         The best of `self.separators` is determined (first found, longest).
         Back slash escaped characters aren't considered as separators