[vector.cons] Use math formatting for 'N' for consistency

Author: jensmaurer

source/containers.tex

@@ -5941,7 +5941,7 @@
 \tcode{last})
 and no reallocations if iterators \tcode{first} and \tcode{last} are of forward, bidirectional, or random access categories.
 It makes order
-\tcode{N}
+$N$
 calls to the copy constructor of
 \tcode{T}
 and order