Fix a typo in a warning message (#2032)

Replaces `impportlib` with `importlib` in the warning message for
`traitsui.__version__`.


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traitsui/__init__.py

@@ -19,7 +19,7 @@ def __getattr__(name):
 
         warn(
             f"traitsui.{name} is deprecated, "
-            f"use impportlib.metadata.version('traitsui') ",
+            f"use importlib.metadata.version('traitsui') ",
             DeprecationWarning,
         )
         return version('traitsui')