Change external link (for robots.txt parser comparison) in docs to a more lasting page

[anubhavp28]

Scrapy documentation (at the end of this section) links to a blog that contains performance numbers for different robots.txt parser backends. It would be better to change the link to point to this issue as it is more lasting. Currently, we are linking to a personal blog website which might disappear if the blog owner decides to change the hosting or blog name.

[eddybruv]

@anubhavp28 please can you review my pull request and tell me if there's any problem with it.

[anubhavp28]

Looks good to me. Scrapy maintainers will review and merge your PR when they will get a chance.