Fix URLs to the prod/staging buildbot master in the doc

[gkistanova]

Thanks for fixing this, Mehdi!

llvm/docs/HowToAddABuilder.rst

16

https://lab.llvm.org/ is more accurate URL. http://lab.llvm.org does a permanent redirect to HTTPS. Or even https://lab.llvm.org/buildbot, since we have the URL for the staging?

18

https://lab.llvm.org/staging

llvm/docs/HowToAddABuilder.rst

 ===================================================================
 How To Add Your Build Configuration To LLVM Buildbot Infrastructure
 ===================================================================
 
 Introduction
 ============
 
 This document contains information about adding a build configuration and
 buildbot-worker to private worker builder to LLVM Buildbot Infrastructure.
 
 Buildmasters
 ============
 
 There are two buildmasters running.
 
-* The main buildmaster at `<http://lab.llvm.org:8011>`_. All builders attached
+* The main buildmaster at `<https://lab.llvm.org/buildbot>`_. All builders attached
   to this machine will notify commit authors every time they break the build.
-* The staging buildbot at `<http://lab.llvm.org:8014>`_. All builders attached
+* The staging buildbot at `<https://lab.llvm.org/staging>`_. All builders attached
   to this machine will be completely silent by default when the build is broken.
   Builders for experimental backends should generally be attached to this
   buildmaster.
 
 Steps To Add Builder To LLVM Buildbot
 =====================================
 Volunteers can provide their build machines to work as build workers to
 public LLVM Buildbot.