RFR: 8259863: doc: JShell snippet doesn't compile

[Jan Lahoda]

Fixing the snippet in `src/jdk.jshell/share/classes/jdk/jshell/package-info.java` so that it compiles.

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Commit messages:
 - 8259863: doc: JShell snippet doesn't compile

Changes: https://git.openjdk.java.net/jdk/pull/3023/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3023&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259863
  Stats: 7 lines in 1 file changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3023.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3023/head:pull/3023

PR: https://git.openjdk.java.net/jdk/pull/3023