Update `help` option in `runTests.py`

Summary:

The help option is slightly out-of-date.

Description:

It currently points to the Math library repo at the end of the help message. That should point to Stan. Perhaps update it to point to the wiki page on testing instead?

Reproducible Steps:

From the command line:

$ ./runTests.py -h

Current Output:

$ ./runTests.py -h

usage: runTests.py [-h] [-j N] [-f F [F ...]] [-d] [-m] [--run-all]
                   tests [tests ...]

Generate and run stan math library tests.

positional arguments:
  tests            The path(s) to the test case(s) to run.
                   Example: 'test/unit', 'test/prob', and/or
                            'test/unit/math/prim/scal/fun/abs_test.cpp'

optional arguments:
  -h, --help       show this help message and exit
  -j N             number of cores for make to use
  -f F [F ...]     Only tests with file names matching these will be executed.
                   Example: '-f chol', '-f gpu', '-f prim mat'
  -d, --debug      request additional script debugging output.
  -m, --make-only  Don't run tests, just try to make them.
  --run-all        Don't stop at the first test failure, run all of them.

See more information at: https://github.com/stan-dev/math

See the last line.

Expected Output:

For the last line, either:

See more information at: https://github.com/stan-dev/stan

or

See more information at: https://github.com/stan-dev/stan/wiki/Testing-Stan-using-Gnu-Make-and-Python

Current Version:

v2.18.0

Open runTests.py and locate the help text shown by ./runTests.py -h, especially its final repository link. Update that link to the Stan repository or the testing wiki page, then run ./runTests.py -h and confirm the displayed help points to the selected destination.