Fix get_dummies dtype docstring

Author: aaronchucarroll

pandas/core/strings/accessor.py

@@ -2387,7 +2387,7 @@ def get_dummies(
             If appending prefix, separator/delimiter to use.
         dummy_na : bool, default False
             Add a column to indicate NaNs, if False NaNs are ignored.
-        dtype: dtype, default int64
+        dtype : dtype, default np.int64
             Data type for new columns. Only a single dtype is allowed.
 
         Returns