RFR: 8261478: InstanceKlass::set_classpath_index does not match comments

[Calvin Cheung]

A simple patch for updating the comment for the `InstanceKlass::set_classpath_index()` function.

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Commit messages:
 - 8261478: InstanceKlass::set_classpath_index does not match comments

Changes: https://git.openjdk.java.net/jdk/pull/4170/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4170&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261478
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4170.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4170/head:pull/4170

PR: https://git.openjdk.java.net/jdk/pull/4170