Update README.md

Change "builder" to "buildx builder"

Co-authored-by: Akihiro Suda <[email protected]>

README.md

@@ -135,7 +135,7 @@ Options:
   --cache-map    The map of actions source to container destination paths for the cache paths
   --scratch-dir  Where the action is stores some temporary files for its processing. Default: 'scratch'
   --skip-extraction  Skip the extraction of the cache from the docker container
-  --builder     The name of the builder. Default: 'default'
+  --builder     The name of the buildx builder. Default: 'default'
   --help         Show this help
 ```