Fix buildArgs -> buildArg (#16)

Documentation showed the `buildArg` input used as `buildArgs`. Very confusing as it's unclear which one is right. Now only one is shown; `buildArg`.

Co-authored-by: Dawid Dziurla <dawidd0811@gmail.com>

Author: dawidd6

README.md

@@ -36,7 +36,7 @@ Build and publish your repository as a Docker image and push it to GitHub Packag
 
 *Optional*. Added to the end of the tag. Useful if you want to let *GitHub Docker* decide the tag, but append something of your own to it.
 
-### `buildArgs`
+### `buildArg`
 
 *Optional*. Any additional build arguments to use when building the image.
 ```yaml

action.yml

@@ -26,7 +26,7 @@ inputs:
   imageTagSuffix:
     description: "Added to the end of the tag."
     required: false
-  buildArgs:
+  buildArg:
     description: "The build arguments for image."
     required: false
 outputs: