sql-statements: change an example in EXPLAIN page from desc to explain

[time-and-fate]

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

According to some feedback, the example on the EXPLAIN page using a desc statement is a bit strange (though they are equivalent). So let's fix it.

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

• master (the latest development version)
• v6.1 (TiDB 6.1 versions)
• v6.0 (TiDB 6.0 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 3ba0ba4

[Mollytonger]

/assign

[Mollytonger]

/remove-translation welcome

[Mollytonger]

/translation doing