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 docs build #1928
memopol

00:02:12

passed qa checks #1930
memopol

00:02:28

passed unit tests #1926
memopol

00:06:36

89.0%
failed docs build #1925
memopol

00:00:03

failed qa checks #1929
memopol

00:03:05

failed qa checks #1927
memopol

00:01:57

failed qa checks #1924
memopol

00:00:06

failed unit tests #1923
memopol

00:00:05