Syntax: Fix & simplify list_item variable
Syntax: Fix & simplify list_item variable

This commit removes an unused capture group from `list_item` variable
and converts preceding lookahead into normal alternation.

Reason:

The `list_item` is only used in lookaheads. Hence it doesn't require
any capture groups. By removing it the missed `)` in it can't do any harm to syntax highlighting of `1) item` list items anymore.

SublimeText-Markdown/MarkdownEditing