Fix typo in create_function_stmt.go comment

Corrected "Undertand" to "Understand" in the TODO comment for the
CreateFunctionStmt struct fields.

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

internal/sql/ast/create_function_stmt.go

@@ -5,7 +5,7 @@ type CreateFunctionStmt struct {
 	Params     *List
 	ReturnType *TypeName
 	Func       *FuncName
-	// TODO: Undertand these two fields
+	// TODO: Understand these two fields
 	Options    *List
 	WithClause *List
 }