Fix marble diagrams urls when releasing

[madgnome]

Urls for marble diagrams updated as part of #1224 have changed to point to master ( https://github.com/reactor/reactor-core/pull/1224#pullrequestreview-124306932 ) .

We need to change them as part of the release process:

• [x] ~Change the urls before release to fix on the commit directly rather than a tag/branch~ N/A
• [x] Change the urls after the release, once the tag is available, to fix on the tag and have "cleaner" urls

Search for https://raw.githubusercontent.com/reactor/reactor-core/master/ to find the urls that need changing.

[simonbasle]

As I'm preparing to release milestone 2, I'll directly fix to the M2 tag right after that and it can be the definitive tag for these urls