Arrow function add a space after fn keyword

[yanggs07]

For configuration or updating questions please read the README and UPGRADE documentation,
or visit: https://gitter.im/PHP-CS-Fixer

When reporting an issue (bug) please provide the following information:

The PHP version you are using ($ php -v):

=> PHP 7.4.1 (cli) (built: Dec 18 2019 14:46:18) ( NTS )

PHP CS Fixer version you are using ($ php-cs-fixer -V):

=> PHP CS Fixer 2.16.1 Yellow Bird by Fabien Potencier and Dariusz Ruminski (c8afb59)

The command you use to run PHP CS Fixer:

=> php-cs-fixer fix foo.php --dry-run --diff

The configuration file you are using, if any:

=> ....................................

If applicable, please provide minimum samples of PHP code (as plain text, not screenshots):

• before running PHP CS Fixer (no changes):

=>             'afterRequest' => fn($data) => trim($data),

• with unexpected changes applied when running PHP CS Fixer:

=>             'afterRequest' => fn ($data) => trim($data),

• with the changes you expected instead:

=>             'afterRequest' => fn($data) => trim($data),

no space after fn would be prefered

[yanggs07]

function_declaration
fixed.