report/text: change delimiter so summary table is valid markdown

This one-character change makes it possible to select istanbul output in a terminal and have it display as a table when pasted directly anywhere markdown is expected (e.g., here, in a github comment).  Example:

File             |   % Stmts |% Branches |   % Funcs |   % Lines |
-----------------|-----------|-----------|-----------|-----------|
   lib\          |     97.06 |     91.58 |        95 |     98.17 |
      npo.src.js |     97.06 |     91.58 |        95 |     98.17 |

Before, I had to manually replace + with |.