Clarify in docs that docker tag doesn't publish

I am attempting to push a tag to a private repository. The documentation for `docker tag`  has an explicit example to for how ["To push an image to a private registry"](https://docs.docker.com/engine/reference/commandline/tag/#tag-an-image-referenced-by-name). My colleague clarified that this command does not in fact push anything, so I thought this PR might save some future novice the same confusion.

Author: jake-lambert-volusion

man/src/image/tag.md

@@ -48,7 +48,7 @@ repository with "version1.0.test":
 
 ## Tagging an image for a private repository
 
-To push an image to a private registry and not the central Docker
+Before pushing an image to a private registry and not the central Docker
 registry you must tag it with the registry hostname and port (if needed).
 
     docker image tag 0e5574283393 myregistryhost:5000/fedora/httpd:version1.0