[hotfix] [docs] Update table environment variable name in common.md

[Huachao]

What is the purpose of the change

Fix enviroment variable name inconsistence in code snippet.

Brief change log

• Change tableEnv to tEnv for both Java and Scala

Verifying this change

Since the changes affect the doc examples and there are no tests. I copied the snippets and verified (compiled) them locally.

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

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

[flinkbot]

CI report:

• 995d861 Azure: SUCCESS

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[davidradl]

I notice that the English version uses tableEnv as the variable name. I suggest changing them both to be consistent in this PR. Changing the variable definition to tableEnv here - would seem to be the minimal change - unless there is a compelling reason to use tEnv.

[davidradl]

Reviewed by Chi on 21/11/24. Asked submitter questions

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