docs: Update doc i18 to flask_babel

[fedepad]

Description

The doc in master regarding i18 still reference the not used anymore by flask_appbuilder fork flask_babelpkg for multiple translations.
This PR fixes that by:

• Updating the i18 doc from references to flask_bablepkg fork as is not used anymore as stated in the version migration
• minor typos encountered along the way of updating i18 doc from flask_bablepkg to flask_babel

[codecov]

Codecov Report

Merging #1792 (590b263) into master (e2d3f01) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1792   +/-   ##
=======================================
  Coverage   77.06%   77.06%           
=======================================
  Files          56       56           
  Lines        8226     8226           
=======================================
  Hits         6339     6339           
  Misses       1887     1887           

Flag	Coverage Δ
python	77.06% <ø> (ø)

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[dpgaspar]

Great, thank you for the contribution