Typo in definition of std::basic_string<...>::assign

[tomalakgeretkal]

In [string.modifiers]/[string::assign] (https://github.com/cplusplus/draft/blob/master/source/strings.tex#L2130):

• \tcode{assign(str.data() + pos rlen)}.
• \tcode{assign(str.data() + pos, rlen)}.