Use correct value for \Ds\Vector::MIN_CAPACITY

[LeonMelis]

PECL's \Ds\Vector::MIN_CAPACITY was updated from 10 to 8 for PECL DS 1.2.0, August 2017 (back in the PHP 5.5 days).

The PHP official documentation was never updated though, so this is likely the cause of these stubs to still use the old value. The official PECL DS polyfill does use the correct value since 2017 already.

See:

• PR for updating PHP documentation: Use correct value for PECL DS MIN_CAPACITY constants php/doc-en#5082
• MIN_CAPACITY value in DS C implementation: https://github.com/php-ds/ext-ds/blob/master/src/ds/ds_vector.h#L12
• MIN_CAPACITY value in DS PHP polyfill: https://github.com/php-ds/polyfill/blob/master/src/Vector.php#L23
• Change from 10 to 8 in 2017 php-ds/polyfill@d370f04