[hotfix][docs] Fix inaccurate description of slot sharing benefit

[shenhuanglin]

What is the purpose of the change

This pull request fixes an inaccurate statement in the Flink Architecture documentation. In the "Task Slots and Resources" section, the description of the Slot Sharing benefit reads "A Flink cluster needs exactly as many task slots as the highest parallelism used in the job." The use of "cluster" implicitly assumes that a cluster runs only a single job, which is incorrect in Session mode where multiple jobs share the same cluster and its task slots. The fix replaces "cluster" with "job" to make the statement accurate regardless of deployment mode (Session, Application, or Per-Job). Both the English and Chinese versions of the page are updated to stay in sync.

Brief change log

• Replaced "A Flink cluster needs exactly as many task slots as the highest parallelism used in the job." with "A Flink job needs exactly as many task slots as the highest parallelism used in the job." in docs/content/docs/concepts/flink-architecture.md.
• Mirrored the same fix in the Chinese translation at docs/content.zh/docs/concepts/flink-architecture.md.

Verifying this change

This change is a trivial documentation fix without any test coverage. It can be verified by rendering the docs locally (docs/build_docs.sh) and inspecting the "Task Slots and Resources" section on both the English and Chinese pages.

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: 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

[flinkbot]

CI report:

• 2016a85 Azure: SUCCESS

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[shenhuanglin]

Thanks for review and merge ! @davidradl