Fix .gitignore to not ignore mongodb-kubernetes dir/file (#410)

# Summary

As part of this PR #21
we changed all the occurrences of old repo to the new repo and in doing
so we ended up adding `mongodb-kubernetes` to the `.gitignore` file.

Becauase of this, if we try to add any file to our new repo in dir
`public/dockerfiles/mongodb-kubernetes` it would be ignored by default.
This PR fixes that. We don't really need that in .gitignore.

## Proof of Work

Create a file in the dir `public/dockerfiles/mongodb-kubernetes` and
make sure that it shows up in `git status`.

Author: viveksinghggits

.gitignore

@@ -65,6 +65,7 @@ multi_cluster/examples/service_account_using_go/service_account_using_go
 _.yaml
 
 # Things we built or download for the e2e tests & local dev
+/mongodb-kubernetes
 multi-cluster-kube-config-creator
 multi-cluster-kube-config-creator_linux
 istio-*