[5.6] Fix typo of missing underscore in not_regexp rule's name

[KisGabo]

Without this fix, properly using "not_regex" rule name in rule arrays will result in error, if using comma in the regexp without quotes, eg.

'field' => 'string|not_regex:/.*, .*/'

The error is: preg_match(): No ending delimiter '/' found
Illuminate/Validation/Concerns/ValidatesAttributes.php:1233

However, these rules work:
'field' => 'string|regex:/.*, .*/'
'field' => 'string|not_regex:"/.*, .*/"'

It's because function parseParameters will try to split the parameter list using str_getcsv() instead of properly returning the whole $parameter.

Since "regex" rule works without quoting the regular expression, but "not_regex" rule doesn't, this behavior is confusing and inconsistent.