Note that build_args has been renamed build-args

Signed-off-by: Dan Lester <dan@danlester.com>

UPGRADE.md

@@ -13,6 +13,7 @@
 * Add [`outputs`](https://github.com/docker/buildx#-o---outputpath-typetypekeyvalue) input
 * Add [`cache-from`](https://github.com/docker/buildx#--cache-fromnametypetypekeyvalue) input (`cache_froms` removed)
 * Add [`cache-to`](https://github.com/docker/buildx#--cache-tonametypetypekeyvalue) input
+* Rename `build_args` input to `build-args` for consistency with other Docker build tools
 * Add `secrets` input
 * Review `tags` input
 * Remove `repository` input. See [Simple workflow](#simple-workflow) for migration