[container.requirements.general] Add missing paragraph number.

Author: zygoloid

source/containers.tex

@@ -523,6 +523,7 @@
 meet the additional requirements of an allocator-aware container, as described in
 \tref{container.alloc.req}.
 
+\pnum
 Given an allocator type \tcode{A}
 and given a container type \tcode{X} having a \tcode{value_type} identical to \tcode{T}
 and an \tcode{allocator_type} identical to \tcode{allocator_traits<A>::rebind_alloc<T>}