web generator: simplify relative link correction from module READMEs when generating module page

[Andros-Spica]

Find passing regular expression that selects fragment while excluding HTML links:

NASSA-modules/website_generator/package_page_template.Rmd

Line 253 in 07dd82e

### correct relative paths

Possible source of solutions; https://stringr.tidyverse.org/articles/regular-expressions.html