[unord.req.general] Insert spaces around + for consistency (#7617)

Author: Andreas-Krug

source/containers.tex

@@ -5903,7 +5903,7 @@
 \indextext{unordered associative containers!requirements}%
 The \tcode{insert}, \tcode{insert_range}, and \tcode{emplace} members
 shall not affect the validity of iterators if
-\tcode{(N+n) <= z * B}, where \tcode{N} is the number of elements in
+\tcode{(N + n) <= z * B}, where \tcode{N} is the number of elements in
 the container prior to the insert operation, \tcode{n} is the
 number of elements inserted, \tcode{B} is the container's bucket count, and
 \tcode{z} is the container's maximum load factor.