Fix error in worker declaration example sniplet

With line:
```
  executable=ctx.attr.worker,
```
I got error like:
```
Error in run: in call to run(), parameter 'executable' got value of type 'Target', want 'File, string, or FilesToRunProvider'
```

site/docs/creating-workers.md

@@ -216,12 +216,13 @@ startup args, the call to `ctx.actions.run` might be:
 ctx.actions.run(
   inputs=ctx.files.srcs,
   outputs=[ctx.attr.output],
-  executable=ctx.attr.worker,
+  executable=ctx.executable.worker,
   mnemonic="someMnemonic",
   execution_requirements={
-    “supports-workers” : “1”,
-    “requires-worker-protocol” : “json},
-  arguments=ctx.attr.args + [“@flagfile”]
+    "supports-workers" : "1",
+    "requires-worker-protocol" : "json",
+  },
+  arguments=ctx.attr.args + ["@flagfile"]
  )
 ```