RFR: 8252399: Update mapMulti documentation to use type test pattern instead of instanceof once JEP 375 exits preview

[Patrick Concannon]

Hi,

Could someone please review my changeset for JDK-8252399: 'Update mapMulti documentation to use type test pattern instead of instanceof once JEP 375 exits preview' ?

This change updates the example code displayed in the API documentation for mapMulti to use the type test pattern introduced in [JEP375](https://openjdk.java.net/jeps/375).

Kind regards,
Patrick

-------------

Commit messages:
 - 8252399: Update mapMulti documentation to use type test pattern instead of instanceof once JEP 375 exits preview

Changes: https://git.openjdk.java.net/jdk/pull/2544/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2544&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252399
  Stats: 68 lines in 2 files changed: 64 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2544.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2544/head:pull/2544

PR: https://git.openjdk.java.net/jdk/pull/2544