[defns.blocked] Use correct character for index subentry.

Author: Eelis

source/lib-intro.tex

@@ -131,7 +131,7 @@
 place a thread in the blocked state
 
 \definition{blocked thread}{defns.blocked}
-\indexdefn{thread, blocked}%
+\indexdefn{thread!blocked}%
 a thread that is waiting for some condition (other than the availability of a processor) to be
 satisfied before it can continue execution\footnote{This definition is taken from POSIX.}