RFR: Fixed bugs in bug filing instructions

[Jesper Wilhelmsson]

Changed to the correct address for filing vulnerabilities and added some clarifications.

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Commit messages:
 - Fixed bugs in bug filing instructions

Changes: https://git.openjdk.java.net/guide/pull/57/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=57&range=00
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/guide/pull/57.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/57/head:pull/57

PR: https://git.openjdk.java.net/guide/pull/57