Fix a stiring issue in the External linter

Summary: See error message in D7170 --  this should be a `.`, not a `,`.

Test Plan:
Faked interprerter and got reasonable error message:

> Unable to locate interpreter "TESTpython2.6" to run linter ArcanistPEP8Linter. You may need to install the intepreter, or adjust your linter configuration.
> TO INSTALL: Install PEP8 using `easy_install pep8`.

This doesn't fix the //real// error, which is that the test should skip if you don't have the interpreter/binary, but that's a little more involved.

Reviewers: hach-que, chad, btrahan

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D7172