Could I ask why with the apostrophe is correct? Google defines "guru" as "a spiritual teacher, especially one who imparts initiation." The article uses the plural of the term. I don't know of any English rules about apostraphizing a plural - generally, I've heard that one should keep the apostrophe for possessives. Could you give me a reference for a rule about adding an apostrophe in a possessive?

Singular: guru; singular possessive: guru's; plural: gurus; plural possessive: gurus'.

(If the word ends with an 's' you can (or not) add only an apostrophe and omit the added 's', afaik it's down to style/how you'd pronounce it. 'Socrates' work' for example, because 'Socrates's' looks and sounds awkward.)

Exactly this. Also, the other points in the list already follow this: "Research your keywords", "Do not pay for SEO courses", etc.

I should have left a comment: thanks for pointing that out, I made the change indeed!

Thank you for doing that. And also thank you for posting the excellent guide to begin with!