Use shipit project as recommended facebookincubator/rebalancer

Summary:
Shipit uses `rebalancer` as project than `facebookincubator/rebalancer`

```
+ /var/www/scripts/opensource/codesync \
+      shipit \
+      --project=rebalancer \
+      --create-new-repo \
+      --source-repo-dir=/data/users/sahilsd/fbsource \
+      --source-branch=. \
+      --skip-source-init \
+      --skip-source-pull \
+      --skip-source-clean \
+      --skip-push \
+      --destination-use-anonymous-https \
+      --create-new-repo-output-path=/data/users/sahilsd/scratch/getdeps/shipit/rebalancer.new
error: invalid value 'rebalancer' for '--project <PROJECT>': Valid projects are specified in the form of '<organization>/<repo_name>', given: rebalancer
```

This matches with the shipit config comment:
https://www.internalfb.com/code/configerator/[8f710a959bf55ae647eb35bcacf3322bbd2ccba2]/source/opensource/shipit_config/facebookincubator/rebalancer.cconf?lines=1

Reviewed By: r-barnes

Differential Revision: D97680878

fbshipit-source-id: 077331584a3e1d58067ca62706efe437e042c36d

Author: sahilsd

build/fbcode_builder/manifests/rebalancer

@@ -1,7 +1,7 @@
 [manifest]
 name = rebalancer
 fbsource_path = fbcode/algopt/rebalancer/
-shipit_project = rebalancer
+shipit_project = facebookincubator/rebalancer
 shipit_fbcode_builder = true
 use_shipit = true