Update `sitmo` pprng main site

[coatless]

c.f. sitmo.com has changed causing

• checking CRAN incoming feasibility ... WARNING

Found the following (possibly) invalid URLs:
URL: http://www.sitmo.com/article/parallel-random-number-generator-in-c/ (moved to https://www.sitmo.com/article/parallel-random-number-generator-in-c/)
From: inst/doc/sitmo_internals.html
Status: 404
Message: Not Found
URL: https://www.sitmo.com/?p=1206
From: DESCRIPTION
man/sitmo-package.Rd
Status: 404
Message: Not Found

[coatless]

Per Thijs (@sitmo / @stdfin)

There is a copy at Github, maybe that’s a good place to declare as the main link?
https://github.com/stdfin/random/