[iterator.requirements.general] Add missing comma.

Author: Eelis

source/iterators.tex

@@ -519,7 +519,7 @@
 \item \libconcept{input_iterator}\iref{iterator.concept.input},
 \item \libconcept{output_iterator}\iref{iterator.concept.output},
 \item \libconcept{forward_iterator}\iref{iterator.concept.forward},
-\item \libconcept{bidirectional_iterator}\iref{iterator.concept.bidir}
+\item \libconcept{bidirectional_iterator}\iref{iterator.concept.bidir},
 \item \libconcept{random_access_iterator}\iref{iterator.concept.random.access},
 and
 \item \libconcept{contiguous_iterator}\iref{iterator.concept.contiguous},