Fix non-existent "@var{thread-id}" in stop reply descriptions

In the description of stop replies, where the "thread" register is
explained, and where the fork and vfork stop reasons are detailed,
there are references to "@var{thread-id}", but such variable does not
actually exist.  This commit fixes it.

Change-Id: If679944aaf15f6f64aabe506339a9e7667864cab