Remove read repo token for installation in README

Since the project is public, the current installation procedure is confusing:

install.packages("remotes")
remotes::install_git("https://package_read:oyj8Yzbryvzw2iGyQQ-N@forge.inrae.fr/umr-g-eau/RADIS.git", build_vignettes = TRUE)

The token should be removed for not bothering the user and to avoid an issue when the token will celebrate its first birthday 😆