[Doc]: Update zh_cn/user_guides/train_test.md

[ththth888]

Motivation

I made this PR to practice my translation skills and get familiar with the procedures of Git.

Modification

Simplified Chinese version of tutorial 4 document translated from its corresponding English version.

Checklist

Submitting this pull request means that,

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.
• CLA has been signed and all committers have signed the CLA in this PR.

[zengyh1900]

lgtm