fix wording

Author: vladak

threads.tex

@@ -642,13 +642,13 @@
 executed in the context of the thread that calls the \funnm{fork}().  Such
 handlers are very useful when \funnm{fork}() is used not only as a wrapper
 around \funnm{exec}().  After \funnm{fork}(), all variables in the child are in
-the state as in the parent, so if a thread not present to the child held a mutex
+the state as in the parent, so if a thread not present in the child held a mutex
 in the parent (see page \pageref{MUTEXES}), the mutex stays locked in the child,
-and trying to lock in the child will lead to a deadlock.  However, if the parent
-locks all the mutexes in the \emph{\texttt{pre-fork}} handler and then unlocks
-them in the \emph{\texttt{post-fork}} handler (both for the parent and the
-child), you will avoid such deadlocks.  That is because when locking mutexes in
-the \emph{\texttt{pre-fork}} handler, other threads are still running so the
+and trying to lock it in the child will lead to a deadlock.  However, if the
+parent locks all the mutexes in the \emph{\texttt{pre-fork}} handler and then
+unlocks them in the \emph{\texttt{post-fork}} handler (both for the parent and
+the child), you will avoid such deadlocks.  That is because when locking mutexes
+in the \emph{\texttt{pre-fork}} handler, other threads are still running so the
 mutexes held by them should be released eventually (usually each thread exits a
 critical section in a short time in well written code).
 \item This scheme will only work if the \emph{\texttt{pre-fork}} handler