Links from GitHub to buildbot.python.org are broken

[mhsmith]

For example, python/cpython#121595 (comment) contains a link to:

https://buildbot.python.org/#builders/1431/builds/376

The correct URL has an extra slash:

https://buildbot.python.org/#/builders/1431/builds/376

[zware]

I thought I had fixed that already, but apparently I missed one :(. Fix incoming.