Fix @param name in the private POP3MessageInfo constructor

[hxperl]

The three-arg POP3MessageInfo constructor documents @param octets, but the parameter is size. The name came from the public two-arg constructor above it, which really does take octets.

$ javadoc -Xdoclint:reference -private -d /tmp/out \
    src/main/java/org/apache/commons/net/pop3/POP3MessageInfo.java
POP3MessageInfo.java:81: error: @param name not found
     * @param octets The size.
              ^

Renames the tag to size; three trailing spaces keep the description column aligned. Clean after the change.

Limits: the constructor is private, so this does not affect the published API docs — it only shows under -private. mvn javadoc:javadoc and mvn test both pass on macOS arm64, JDK 17 (632 tests, 0 failures).

• I used AI to create any part of, or all of, this pull request. Written with Claude Code (Claude Opus 5); it found the mismatch, made the one-line edit, and ran the build.