gdb: update comment in code_breakpoint::re_set_default

Spotted a comment in code_breakpoint::re_set_default that was added in
commit:

  commit 6cce025114ccd0f53cc552fde12b6329596c6c65
  Date:   Fri Mar 3 19:03:15 2023 +0000

      gdb: only insert thread-specific breakpoints in the relevant inferior

that was incorrect.  The comment was not updated to take inferior
specific breakpoints into account.

This commit just updates the comment, there's no user visible changes
after this commit.