change Error message on merge type

Author: KevsterAmp

pandas/core/reshape/merge.py

@@ -985,7 +985,10 @@ def __init__(
         # GH 59435: raise when "how" is not a valid Merge type
         merge_type = {"left", "right", "inner", "outer", "cross", "asof"}
         if how not in merge_type:
-            raise ValueError(f"'{how}' is not a valid Merge type {merge_type}")
+            raise ValueError(
+                f"'{how}' is not a valid Merge type: "
+                f"left, right, inner, outer, cross, asof"
+            )
 
         self.left_on, self.right_on = self._validate_left_right_on(left_on, right_on)