[thread.condition.general] Fix style of quotation marks

Author: jensmaurer

source/threads.tex

@@ -4183,7 +4183,7 @@
 \pnum
 The implementation behaves as if all executions of \tcode{notify_one}, \tcode{notify_all}, and each
 part of the \tcode{wait}, \tcode{wait_for}, and \tcode{wait_until} executions are
-executed in a single unspecified total order consistent with the "happens before" order.
+executed in a single unspecified total order consistent with the ``happens before'' order.
 
 \pnum
 Condition variable construction and destruction need not be synchronized.