fix: bug in the parsing of balanced expressions

details:
- a balanced expression is of the form
  - `(expr)`
  - `((expr))`
  - `([(expr)])`
  - etc.
- balanced expression with more than 2 consecutive identical tokens
  were not correctly parsed

modified:   omd_parser.ml

related to case D of https://github.com/ocaml/omd/issues/10#issuecomment-60100329