Fix typos, etc.

[sdruskat]

From Axel Loewe's PubPeer comment:

• p.3 users' -> user's
• p.6 consider naming SPARQL together with the reference
• p.8 catalogue -> catalogues
• section 3.3 There are a couple of extra commas in point 1
• section 3.4.2 appropriatly -> appropriately
• section 3.4.2 Is there a comma missing after "publication repositories APIs"?

Misc

• Acknowledge PubPeer reviewers in acknowledgments section