docs: fx dry-run usage (#117)

### 📖 Summary

fix usage of `dry-run` option in docs example. It is supposed to be `dry-run` and not `dry_run`.

See the current docs here: https://woodpecker-ci.org/plugins/Docker%20Buildx for an example

### 📑 Build PR Images?

PR images are not needed

### 💬 Details

_No response_

Reviewed-on: https://codeberg.org/woodpecker-plugins/docker-buildx/pulls/117
Reviewed-by: Lauris BH <lafriks@noreply.codeberg.org>
Co-authored-by: bretello <bretello@distruzione.org>
Co-committed-by: bretello <bretello@distruzione.org>

docs.md

@@ -85,7 +85,7 @@ docker-build:
   settings:
     repo: codeberg.org/${CI_REPO_OWNER}/hello
     registry: codeberg.org
-    dry_run: true
+    dry-run: true
     output: type=oci,dest=${CI_REPO_OWNER}-hello.tar
 ```
 
@@ -187,7 +187,7 @@ steps:
       - <<: *proxy_conf
     privileged: true
     settings:
-      dry_run: true
+      dry-run: true
       repo: *publish_repos
       dockerfile: Dockerfile.multi
       platforms: linux/amd64