Merge pull request #8 from docker/readme-fix

Correct example tag_with_sha

Author: zappy-shu

README.md

@@ -131,7 +131,7 @@ Then the image will be tagged with:
 myregistry/myorg/myimage:foo
 myregistry/myorg/myimage:bar
 myregistry/myorg/myimage:lastest
-myregistry/myorg/myimage:c6df8c6
+myregistry/myorg/myimage:sha-c6df8c6
 ```
 
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