[algorithms.parallel.defns] Avoid 'must'

Author: jensmaurer

source/algorithms.tex

@@ -335,7 +335,7 @@
 another function invocation is specified to synchronize with it,
 and if it is not a memory allocation or deallocation function.
 \begin{note}
-Implementations must ensure that internal synchronization
+Implementations ensure that internal synchronization
 inside standard library functions does not prevent forward progress
 when those functions are executed by threads of execution
 with weakly parallel forward progress guarantees.