[MRG] Tiny fix in SSNB

[eloitanguy]

Types of changes

Added a forgotten factor for the computation of the barycentric mapping (initialisation for SSNB)
Removed the cell about the barycentric mapping in the SSNB example: it had the same typo and is unnecessary.

How has this been tested (if it applies)

Still pytest covered + reran example for illustration.

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

Merging #535 (b3578f0) into master (8a4a5a6) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #535   +/-   ##
=======================================
  Coverage   96.31%   96.31%           
=======================================
  Files          67       67           
  Lines       14136    14136           
=======================================
  Hits        13615    13615           
  Misses        521      521