[hotfix] [docs] Fix typo in concepts.

[echauchot]

What is the purpose of the change

This PR fixes minor typos in the concepts part of the website

Brief change log

cf above

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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

no, no side effect

Documentation

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

R: @aljoscha

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Automated Checks

Last check on commit f4b5ddd (Fri Apr 10 15:10:00 UTC 2020)

Warnings:

• Documentation files were touched, but no .zh.md files: Update Chinese documentation or file Jira ticket.

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Review Progress

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

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CI report:

• f4b5ddd Travis: SUCCESS Azure: SUCCESS

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