fix fn in ecb-cipher

[9iang22]

Problem:
The rule has an FN case:

  patterns:
  - pattern: |
      Cipher $VAR = $CIPHER.getInstance($MODE); // should be "$MODE"
  - metavariable-regex:
      metavariable: $MODE
      regex: .*ECB.*

It is unable to match situations like:

    String algo = "AES/ECB/NoPadding";
    Cipher c = Cipher.getInstance(algo);  // Incorrectly matches on 'algo', which is the variable name

The fix corrects the usage of $MODE to "$MODE".