[Fix] Fix the cross-codebase reference problem in metafile README

[ZCMax]

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Motivation

We need to support README: https://github.com/open-mmlab/mmdetection/blob/master/configs/cascade_rcnn/README.md.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the back-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #1644 (5a4722e) into dev (7a31b76) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1644   +/-   ##
=======================================
  Coverage   50.15%   50.15%           
=======================================
  Files         223      223           
  Lines       19124    19124           
  Branches     3141     3141           
=======================================
  Hits         9591     9591           
  Misses       8953     8953           
  Partials      580      580           

Flag	Coverage Δ
unittests	50.15% <ø> (ø)

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