Fix SpaceBeforeFirstArg cop for multiline arguments

I have this code:

````ruby
SimpleCov.formatter = SimpleCov::Formatter::MultiFormatter[
    SimpleCov::Formatter::HTMLFormatter,
    SimpleCov::Formatter::RcovFormatter
]
````

The cop treats everything after the equal sign as `arg1_with_space` and this does in fact start with a space, but the regular expression searches for any line not starting with a whitespace, not just the first one.