Fix deploy 404s

[chalin]

• Contributes to Top 404s as of 2022/03/04 #692 for the deploy path
• This is the resulting diff in the generated _redirects file, when we ignore changes in whitespace:

  $ (cd public && git diff -bw --ignore-blank-lines)

  diff --git a/_redirects b/_redirects
  index 5d40ff6..07b3c0a 100644
  --- a/_redirects
  +++ b/_redirects
  @@ -11,5 +11,5 @@
   /docs/operate/*                 /docs/2.6/operate/:splat
   /concepts                       /docs/2.6/concepts     301!
   /docs/concepts/*                /docs/2.6/concepts/:splat
  -/deploy     /docs/2.6deploy     301!
  +/deploy                         /docs/2.6/deploy     301!
   /docs/deploy/*                  /docs/2.6/deploy/:splat

  Note the added /.

Redirect test:

• https://deploy-preview-708--keda.netlify.app/deploy
  vs.
  https://keda.sh/deploy (before this PR is merged, this results in a 301 then a 404)

[netlify]

✅ Deploy Preview for keda ready!

🔨 Explore the source changes: 9de0419

🔍 Inspect the deploy log: https://app.netlify.com/sites/keda/deploys/6239bd65b5ead700086fe2af

😎 Browse the preview: https://deploy-preview-708--keda.netlify.app

[chalin]

@JorTurFer @tomkerkhove - This is ready for final review and merge. Thanks.