RFR: Improve mermaid-filter installation instructions

[Magnus Ihse Bursie]

mermaid-filters depends on puppeteer, which can fail to install on linux without --unsafe-perm=true.

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Commit messages:
 - Improve mermaid-filter installation instructions

Changes: https://git.openjdk.java.net/guide/pull/50/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=50&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/guide/pull/50.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/50/head:pull/50

PR: https://git.openjdk.java.net/guide/pull/50