Swift: Fix typo in example.

Author: geoffw0

swift/ql/src/queries/Security/CWE-089/SqlInjectionGood.swift

@@ -1,4 +1,4 @@
 let safeQuery = "SELECT * FROM users WHERE username=?"
 
 let stmt = try db.prepare(safeQuery, userControlledString) // GOOD
-try stmt2.run()
+try stmt.run()