Update link to Writing R Extensions

[PeteHaitch]

At least for me, the link in this section didn't properly resolve

pkgrevdocs/docs/package-review-checklist.md

Line 128 in 6a84475

- [ ] C function registration used (See [Registering native routines](http://cran.fhcrc.org/doc/manuals/R-exts.html#Registering-native-routines).

It might be replaced with one of:

• Current R(?): https://cran.r-project.org/doc/manuals/R-exts.html#Registering-native-routines
• R-devel: https://cran.r-project.org/doc/manuals/r-devel/R-exts.html#Registering-native-routines

Newer, arguably prettier, formatting is currently applied to the R-devel version.

Also, the current dot point is missing a final closing parenthesis.

[yush-1018]

Hi @PeteHaitch
"I have updated the broken link to 'Writing R Extensions' in the checklist and added the missing closing parenthesis as requested. I've raised a PR with the changes."