Fix typo, formulas typo

Pascal Engélibertrequested to merge
tuxmain/duniter-whitepaper:master into master

Fix:

  • English typo (no space before punctuations :;?!)
  • et.al -> et al.
  • Parentheses in formula 4.1
  • D U -> DU in formula 1.1
  • Colon in Sources title (there must not be colons at the end of titles)

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