fix broken urls in guidelines

[sanette]

page:

https://ocamlverse.github.io/content/documentation_guidelines.html

in this paragraph, both links are broken.

An example would be Simon Cruanes’ sequence documentation, where the elements are neatly organized into sections, such as “Build a sequence”, “Consume a sequence”, etc. Another example is Daniel Bünzli’s Fmt documentation.

[bluddy]

Thanks for noticing! Fixed.