fix jgit javadoc link

[JonnyJD]

Unfortunately there is no generic javadoc link anymore.
I linked the latest stable version.
Old version docs are not removed so this should keep working.

The previous link leads to a project page and there is no link to the javadoc on that page.

[YueLinHo]

As time goes by the link is changed.
Perhaps using http://www.eclipse.org/jgit/documentation/, something like this:

The official JGit API documentation can be found at 
http://www.eclipse.org/jgit/documentation/[].

[ben]

Yeah, if we can't point to an always-the-latest URL, we should drop the reader at a place where that can be found.

[ben]

@JonnyJD, want to make the change we suggested and push a new commit to this branch?

[JonnyJD]

Sorry, I have somewhat this PR. I added the commit as suggested by @YueLinHo

[ben]

✨ Thanks!