docs: update irc channel

According to the [virglrenderer README.rst][1] file, #virgil3d has moved
from FreeNode to OFTC. Let's update our link as well.

While we're at it, use a proper link as well.

[1]: https://gitlab.freedesktop.org/virgl/virglrenderer/-/blob/master/README.rst
2 jobs for !13836 with docs-virgl in 2 minutes and 56 seconds (queued for 26 seconds)
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  Sanity
passed sanity #16166831

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  Deploy
passed test-docs-mr #16166832

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