[algorithms.parallel.user] remove stray full stop

Fixes NB JP 12 (C++17 DIS)

Author: tkoeppe

source/algorithms.tex

@@ -1284,7 +1284,7 @@
 these parallel algorithms that could be formed by the invocation with the
 specified default predicate or operation (where applicable) shall not directly
 or indirectly modify objects via their arguments, nor shall they rely on the
-identity of the provided objects..
+identity of the provided objects.
 
 \rSec2[algorithms.parallel.exec]{Effect of execution policies on algorithm execution}