Fix typo: s/wepbacker/webpacker

Author: paulca

lib/webpacker/manifest.rb

@@ -47,7 +47,7 @@ def handle_missing_entry(name)
     def missing_file_from_manifest_error(bundle_name)
       msg = <<-MSG
 Webpacker can't find #{bundle_name} in #{config.public_manifest_path}. Possible causes:
-1. You want to set wepbacker.yml value of compile to true for your environment
+1. You want to set webpacker.yml value of compile to true for your environment
    unless you are using the `webpack -w` or the webpack-dev-server.
 2. Webpack has not yet re-run to reflect updates.
 3. You have misconfigured Webpacker's config/webpacker.yml file.