Fix 0.15 stable docs

[NicolasHug]

Fixes #7420
Just some cosmetics:

• Torchvision main documentation -> Torchvision 0.15 documentation
• main (0.15.1a0+42759b1 ) -> 0.15

Now docs properly look like:

with the correct title in the header

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/7425

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[pmeier]

Stamping, thanks Nicolas!