[SPARK-37032][SQL] Fix broken SQL syntax link in SQL Reference page

### What changes were proposed in this pull request?
Current 4 link about SQL syntax under SQL Reference page is broken, this pr will fix the broken link. And re-link to the subsection under SQL Syntax doc

![image](https://user-images.githubusercontent.com/46485123/137661935-d303ef9f-3596-4cac-896d-a53d19c1ca97.png)

### Why are the changes needed?
Fix SQL Reference doc's broken link

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Not need

Closes #34307 from AngersZhuuuu/SPARK-37032.

Authored-by: Angerszhuuuu <angers.zhu@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

docs/sql-ref.md

@@ -34,7 +34,7 @@ Spark SQL is Apache Spark's module for working with structured data. This guide
  * [Literals](sql-ref-literals.html)
  * [Null Semantics](sql-ref-null-semantics.html)
  * [SQL Syntax](sql-ref-syntax.html)
-   * [DDL Statements](sql-ref-syntax-ddl.html)
-   * [DML Statements](sql-ref-syntax-dml.html)
-   * [Data Retrieval Statements](sql-ref-syntax-qry.html)
-   * [Auxiliary Statements](sql-ref-syntax-aux.html)
+   * [DDL Statements](sql-ref-syntax.html#ddl-statements)
+   * [DML Statements](sql-ref-syntax.html#dml-statements)
+   * [Data Retrieval Statements](sql-ref-syntax.html#data-retrieval-statements)
+   * [Auxiliary Statements](sql-ref-syntax.html#auxiliary-statements)