[example] update RGCN README

[Rhett-Ying]

Description

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• I've leverage the tools to beautify the python and c++ code.
• The PR is complete and small, read the Google eng practice (CL equals to PR) to understand more about small PR. In DGL, we consider PRs with less than 200 lines of core code change are small (example, test and documentation could be exempted).
• All changes have test coverage
• Code is well-documented
• To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
• Related issue is referred in this PR
• If the PR is for a new model/paper, I've updated the example index here.

Changes

[dgl-bot]

To trigger regression tests:

• @dgl-bot run [instance-type] [which tests] [compare-with-branch];
  For example: @dgl-bot run g4dn.4xlarge all dmlc/master or @dgl-bot run c5.9xlarge kernel,api dmlc/master

[dgl-bot]

Commit ID: 59581690a51e61a676e8ed70fef56aeae1739902

Build ID: 1

Status: ✅ CI test succeeded.

Report path: link

Full logs path: link

[mfbalin]

If we enable GPU sampling for this example as in #6958, the GPU runtimes should go down a lot more. Let's consider doing that or at least adding the option before the release?

[Rhett-Ying]

@mfbalin if new mode is added, you'd better update the README accordingly.

[mfbalin]

@mfbalin if new mode is added, you'd better update the README accordingly.

I don't currently have the computational resources to run the mag240M example and update the numbers there. Could I ask you or anyone on the team to do that? That would be a big help.