Parser Stream Operations documentation references wrong Stream types

### Version of Beast
1.70

### Steps necessary to reproduce the problem
Open
https://www.boost.org/doc/libs/1_70_0/libs/beast/doc/html/beast/using_http/parser_stream_operations.html

## Problem:
5 out of 6 read functions listed in Table 1.22, _Parser Stream Operations_, refer to `WriteStream`, while they should refer to `ReadStream`.