Fix zuul.artifacts example in documentation

zuul.artifacts example fails to be parsed with an Ansible parse error:

  ERROR! Syntax Error while loading YAML.  mapping values are not
  allowed in this context

Change-Id: I7d344e96de74c3b55ee7430ae391d344b2485c2d

doc/source/reference/jobs.rst

@@ -842,7 +842,7 @@ under the :var:`zuul.artifacts` dictionary.  For example:
                 url: http://example.com/path/to/package.tar.gz
                 metadata:
                   version: 3.0
-              - name: docs:
+              - name: docs
                 url: build/docs/
 
 If the value of **url** is a relative URL, it will be combined with