docs: correct syntax for sql-stmt-create-materialized-view.md

[congxuebin]

Synopsis change, according to #47

closes: #ISSUE_Number

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[TomShawn]

LGTM

[tuhaihe]

LGTM.

[tuhaihe]

Do we need to update the line 65 as well?

https://github.com/cloudberrydb/cloudberrydb-site/blob/c56e3ef72d42380f516a3fe3f3a67c2a546dce93/docs/sql-stmts/sql-stmt-create-materialized-view.md?plain=1#L65

[TomShawn]

Do we need to update the line 65 as well?

https://github.com/cloudberrydb/cloudberrydb-site/blob/c56e3ef72d42380f516a3fe3f3a67c2a546dce93/docs/sql-stmts/sql-stmt-create-materialized-view.md?plain=1#L65

Updated in eb2d9a7