Fix git commit blog article link

The blog article linked in the README misses the trailing forward slash
which produces a 404 error instead of redirecting to the correct URL.

README.md

@@ -116,7 +116,7 @@ To make the process as seamless as possible, we ask for the following:
      * Have test cases for the new code. If you have questions about how to do it, please ask in your pull request.
      * Run `go fmt`
      * Squash your commits into a single commit. `git rebase -i`. It’s okay to force update your pull request.
-     * **Write a good commit message.** This [blog article](http://chris.beams.io/posts/git-commit/) is a good resource for learning how to write good commit messages, the most important part being that each commit message should have a title/subject in imperative mood starting with a capital letter and no trailing period: *"Return error on wrong use of the Paginator"*, **NOT** *"returning some error."* Also, if your commit references one or more GitHub issues, always end your commit message body with *See #1234* or *Fixes #1234*. Replace *1234* with the GitHub issue ID. The last example will close the issue when the commit is merged into *master*.
+     * **Write a good commit message.** This [blog article](http://chris.beams.io/posts/git-commit) is a good resource for learning how to write good commit messages, the most important part being that each commit message should have a title/subject in imperative mood starting with a capital letter and no trailing period: *"Return error on wrong use of the Paginator"*, **NOT** *"returning some error."* Also, if your commit references one or more GitHub issues, always end your commit message body with *See #1234* or *Fixes #1234*. Replace *1234* with the GitHub issue ID. The last example will close the issue when the commit is merged into *master*.
      * Make sure `go test ./...` passes, and `go build` completes. Our [Travis CI loop](https://travis-ci.org/spf13/hugo) (Linux and OS X) and [AppVeyor](https://ci.appveyor.com/project/spf13/hugo/branch/master) (Windows) will catch most things that are missing.
 
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