[FLINK-11892] [docs] Refine README of end-to-end tests to make it more clear

[carp84]

What is the purpose of the change

• This PR is a document refinement for end-to-end tests.

Brief change log

• Correct the markdown syntax error so as to show the flink dir meaning clearly.
• Add note to prevent user run the nightly script with explicit bash command.

Verifying this change

This change is a document refinement without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

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Review Progress

• ✅ 1. The [description] looks good.
  • Approved by @sunjincheng121 [committer]
• ✅ 2. There is [consensus] that the contribution should go into to Flink.
  • Approved by @sunjincheng121 [committer]
• ❓ 3. Needs [attention] from.
• ✅ 4. The change fits into the overall [architecture].
  • Approved by @sunjincheng121 [committer]
• ✅ 5. Overall code [quality] is good.
  • Approved by @sunjincheng121 [committer]

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[sunjincheng121]

@flinkbot approve description
@flinkbot approve consensus

[sunjincheng121]

Thanks for the quick and nice fix! @carp84
The changes LGTM.
Merging ...
@flinkbot approve all

[carp84]

Thanks for review @sunjincheng121 !