Update number of hosting websites

Dependencies

• [2] BZTIJPUB Finish the predictions
• [3] CBUCBYTV Progress
• [4] 73PQOETU Update slides to 2021 situation
• [*] ARAFRHKU Create the outline of the talk

Change contents

• replacement in sections/7-predictions.md at line 96
  [2.649]→[2.649:713](∅→∅)
  − Wikipedia lists 16 different hosting websites that support Git.
  [2.649]

  [2.713]
  + Wikipedia lists 19 different hosting websites that support Git.