fix displaying of ArgumentException (vyperlang#3500)

The change is pretty self-explanatory.
```vyper
@internal
@view
def bar():
    pass

@external
def foo():
    self.bar(12)
```
Was failing to compile with:
`vyper.exceptions.ArgumentException: Invalid argument count for call to 'bar': expected 0 to 0, got 1`

And now fail to compile with:

`vyper.exceptions.ArgumentException: Invalid argument count for call to 'bar': expected 0, got 1`

Co-authored-by: Tanguy Rocher <tanguy.rocher@protonmail.com>

vyper/ast/validation.py

@@ -48,7 +48,7 @@ def validate_call_args(
             arg_count = (arg_count[0], 2**64)
 
         if arg_count[0] == arg_count[1]:
-            arg_count == arg_count[0]
+            arg_count = arg_count[0]
 
     if isinstance(node.func, vy_ast.Attribute):
         msg = f" for call to '{node.func.attr}'"