fix documentation for the --count option

It erroneously claimed that it set the application exit code to 1 if the
count was greater than 1. However, this is false, because the --count
option doesn't modify the error code at any time.

If the count was greater than 1, then the exit code was already 1, even
in the absence of --count, unless --exit-zero was used.

This documentation bug resulted in people reading the `flake8 --help`
output and believing that --count is mandatory in order to ensure that
flake8 produces errors in automated processes (such as CI scripts) when
flake8 violations are detected.