Fix deprecated way of calling ERB.new in Ruby 3.1

See https://bugs.ruby-lang.org/issues/14256 - `eoutvar` is supposed to
be passed as a keyword argument now.

In Ruby 3.1 (but probably since some earlier version than that), the old
way was triggering a deprecation warning.

_build/build-html

@@ -457,7 +457,7 @@ class FormatPagesGenerator
 
   def erb(name)
     bin_dir = File.expand_path(File.dirname(__FILE__))
-    ERB.new(File.read("#{bin_dir}/#{name}.html.erb"), nil, nil, "_erbout_#{name}")
+    ERB.new(File.read("#{bin_dir}/#{name}.html.erb"), eoutvar: "_erbout_#{name}")
   end
 end