[allocator.requirements] Fix pointer_to expression

Fixes #1592

Author: jwakely

source/lib-intro.tex

@@ -1891,9 +1891,9 @@
   \tcode{X::const_pointer}                &
   \tcode{static_cast<} \tcode{X::const_pointer\brk{}>(x) == q} & \\ \rowsep
 
-\tcode{pointer_traits<\brk{}X::pointer\brk{}>::pointer_to(r)}  &
+\tcode{pointer_traits<\brk{}X::pointer\brk{}>::pointer_to(*p)}  &
   \tcode{X::pointer}        &
-                            & \\ \rowsep
+  same as \tcode{p}         & \\ \rowsep
 
 \tcode{a.allocate(n)}   &   \tcode{X::pointer}  &
 Memory is allocated for \tcode{n} objects of type \tcode{T} but objects