Fix 404 error on banniere-en.png

Filename was incorrect and was using an underscore instead of a dash.
8 jobs for 201-homepage-hit-a-404-trying-to-load-banniere_en-png in 11 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #1928
memopol
docs build

00:02:12

passed #1930
memopol
qa checks

00:02:28

passed #1926
memopol
unit tests

00:06:36

89.0%
failed #1925
memopol
docs build

00:00:03

failed #1924
memopol
qa checks

00:00:06

failed #1927
memopol
qa checks

00:01:57

failed #1929
memopol
qa checks

00:03:05

failed #1923
memopol
unit tests

00:00:05