C++: Increase the query precision.

Author: geoffw0

cpp/ql/src/Security/CWE/CWE-311/CleartextFileWrite.ql

@@ -5,7 +5,7 @@
  * @kind problem
  * @problem.severity warning
  * @security-severity 7.5
- * @precision medium
+ * @precision high
  * @id cpp/cleartext-storage-file
  * @tags security
  *       external/cwe/cwe-313