Missed self.

Missed `self.` during making PR, tests failed. Now corrected!

Author: dimithras

pandas/core/groupby/grouper.py

@@ -524,8 +524,8 @@ def __init__(
                     raise ValueError(f"Grouper for '{t}' not 1-dimensional")
                 self.grouper = self.index.map(self.grouper)
 
-                if isinstance(grouper, MultiIndex):
-                    self.grouper = grouper._values
+                if isinstance(self.grouper, MultiIndex):
+                    self.grouper = self.grouper._values
 
                 if not (
                     hasattr(self.grouper, "__len__")