Use spaces instead of tabs in config example

[robertfausk]

of phpstan.neon cause tabs are not allowed and will result in an error after c&p this example and running phpstan.

Error message example:

Note: Using configuration file /var/www/html/phpstan.neon.dist.

In Decoder.php line 357:
                                                               
  Invalid combination of tabs and spaces on line 7, column 9. 

[ondrejmirtes]

In NEON tabs are allowed but you can't mix them with spaces.

[ondrejmirtes]

So it doesn't make sense to merge this, someone might bump into an opposite issue.

[robertfausk]

What do you think about leaving a hint next to this example that copy & paste can lead to an error (as happened to me)?

[ondrejmirtes]

I think the message is self-evident and is easy to fix so there doesn’t have to be a notice in the README.