[Doc] Fix invalid symbols in docstring

[MeowZheng]

Motivation

There exits some invalid symbols in docstring, and raise exception when compiling documents with latex

Modification

1. revise the docstring in mmcv/ops/correlation.py
2. revise the docstring in mmcv/ops/group_points.py
3. revise the docstring in mmcv/ops/points_sampler.py

BC-breaking (Optional)

No

Use cases (Optional)

No

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.