Fix DisallowSpaceIndentSniff to allow top-level PHP DocBlocks

The sniff is currently confused when there is a single space used at the beginning of the line before the `*` in a DocBlock.
Please do this:

1. remove space after `(` and before `)` to match PHPCS coding standard
2. replace `strlen( $content ) === 1` with `$content === ' '` to clearly indicate that expect single space