chore: minor changes to show cluster syntax

[yezizp2012]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

As title and suggested by @neverchanje , changed from SHOW CLUSTERS to SHOW CLUSTER.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
• All checks passed in ./risedev check (or alias, ./risedev c)
• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

• My PR contains critical fixes that are necessary to be merged into the latest release. (Please check out the details)

Documentation

• My PR contains user-facing changes.

Click here for Documentation

Types of user-facing changes

Please keep the types that apply to your changes, and remove the others.

• Installation and deployment
• Connector (sources & sinks)
• SQL commands, functions, and operators
• RisingWave cluster configuration changes
• Other (please specify in the release note below)

Release note

[neverchanje]

You may want to merge it into the v1.0 branch to avoid claiming the breaking change in our next version. It'd still be fine if so.

[yezizp2012]

You may want to merge it into the v1.0 branch to avoid claiming the breaking change in our next version. It'd still be fine if so.

Yes sure, I added the need-cherry-pick-v1.0 label for it.

[codecov]

Codecov Report

Merging #10932 (437d265) into main (88ecdf8) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main   #10932      +/-   ##
==========================================
- Coverage   69.92%   69.92%   -0.01%     
==========================================
  Files        1309     1309              
  Lines      223841   223841              
==========================================
- Hits       156515   156511       -4     
- Misses      67326    67330       +4     

Flag	Coverage Δ
rust	69.92% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sqlparser/src/ast/mod.rs	85.03% <0.00%> (ø)
src/sqlparser/src/keywords.rs	100.00% <ø> (ø)
src/sqlparser/src/parser.rs	86.06% <ø> (ø)

... and 7 files with indirect coverage changes

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