[thread.sharedtimedmutex.requirements.general] Fix typo

Author: timsong-cpp

source/threads.tex

@@ -2658,7 +2658,7 @@
 shared mutex types\iref{thread.sharedmutex.requirements}, and additionally
 meet the requirements set out below. In this description,
 \tcode{m} denotes an object of a shared timed mutex type,
-\tcode{rel_type} denotes an object of an instantiation of
+\tcode{rel_time} denotes an object of an instantiation of
 \tcode{duration}\iref{time.duration}, and
 \tcode{abs_time} denotes an object of an instantiation of
 \tcode{time_point}\iref{time.point}.