[format.parse.ctx] add comma before consisting

[eisenwave]

It is more correct to write a comma here. The current wording suggests that

An instance of basic_fomat_parse_context holds the one format string parsing state (of many parsing states) which happens to consist of ...

What the sentence is actually trying to express is

An instance of basic_format_parse_context holds the the format string parsing state; this state consists of ...