[hotfix][docs] Fix typo in alter.md

[potterxu]

What is the purpose of the change

This pull request aims at fixing typo in documents

Brief change log

• Fix typos in alter.md

Verifying this change

N/A

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

N/A

Documentation

N/A

[flinkbot]

CI report:

• b830e4ae667a57c4fd568792990577af6b6be8ba Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[potterxu]

Hi @swuferhong @luoyuxia , would you mind take a look at the above changes in doc? Thank you.

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