Apply suggestions from code review

Co-authored-by: Johel Ernesto Guerrero Peña <[email protected]>

Author: jensmaurer

source/containers.tex

@@ -1580,7 +1580,7 @@
 \pnum
 \effects
 Inserts copies of elements in \tcode{rg} before \tcode{p}.
-Each iterator in the range \tcode{rg} shall be dereferenced exactly once.
+Each iterator in the range \tcode{rg} is dereferenced exactly once.
 
 \pnum
 \returns
@@ -1702,7 +1702,7 @@
 referring to the elements of \tcode{a}.
 For \tcode{vector} and \tcode{deque},
 also invalidates the past-the-end iterator.
-Each iterator in the range \range{i}{j} shall be dereferenced exactly once.
+Each iterator in the range \range{i}{j} is dereferenced exactly once.
 \end{itemdescr}
 
 \indexcont{assign_range}%
@@ -1737,7 +1737,7 @@
 referring to the elements of \tcode{a}.
 For \tcode{vector} and \tcode{deque},
 also invalidates the past-the-end iterator.
-Each iterator in the range \tcode{rg} shall be dereferenced exactly once.
+Each iterator in the range \tcode{rg} is dereferenced exactly once.
 \end{itemdescr}
 
 \begin{itemdecl}