[alg.transform] Fix comma that should be a full stop.

Author: tkoeppe

source/algorithms.tex

@@ -7079,7 +7079,7 @@
 \expects
 For parallel algorithm overloads
 \tcode{op} and \tcode{binary_op} satisfy the requirements
-specified in \ref{algorithms.parallel.user},
+specified in \ref{algorithms.parallel.user}.
 \tcode{op} and \tcode{binary_op} do not invalidate iterators or subranges, nor
 modify elements in the ranges
 \begin{itemize}