[time.clock.cast.fn] Fix inappropriate phrasing of "Mandates:" element.

Author: zygoloid

source/time.tex

@@ -3798,9 +3798,8 @@
 
 \pnum
 \mandates
-If, among the well-formed clock time conversion expressions
-from the above list, there is not a unique best expression,
-the \tcode{clock_cast} is ambiguous and the program is ill-formed.
+Among the well-formed clock time conversion expressions
+from the above list, there is a unique best expression.
 
 \pnum
 \returns