[char.traits.require] use nullptr for null pointer constant

Author: jwakely

source/strings.tex

@@ -151,7 +151,7 @@
 \tcode{X::find(p,n,c)}  &   \tcode{const X::char_type*} &
  \returns
 the smallest \tcode{q} in \tcode{[p,p+n)} such that
-\tcode{X::eq(*q,c)} is \tcode{true}, zero otherwise.                        &   linear      \\ \rowsep
+\tcode{X::eq(*q,c)} is \tcode{true}, \tcode{nullptr} otherwise.                &   linear      \\ \rowsep
 \tcode{X::move(s,p,n)}  &   \tcode{X::char_type*}   &
 for each \tcode{i} in \tcode{[0,n)}, performs \tcode{X::assign(s[i],p[i])}.
 Copies correctly even where the ranges \tcode{[p,p+n)} and \tcode{[s,s+n)} overlap.\br \returns \tcode{s}.    &   linear  \\ \rowsep