Label bot: rename A:dataframe to A:plugin-polars (nushell#16612)

## Release notes summary - What our users need to know
N/A

Author: 132ikl

.github/labeler.yml

@@ -1,7 +1,7 @@
 # A bot for automatically labelling pull requests 
 # See https://github.com/actions/labeler
 
-A:dataframe:
+A:plugin-polars:
   - changed-files:
     - any-glob-to-any-file:
       - crates/nu_plugin_polars/**