JS: Fix Opal build to not mess with backslashes

The problem is that the replacement in `String#sub(pattern, replacement)`
is interpreted by Ruby:

    If replacement is a String that looks like a pattern's capture group
    but is actually not a pattern capture group e.g. "\'", then it will
    have to be preceded by two backslashes like so "\\'".

1 files changed, 1 insertions, 1 deletions

diff --git a/Rakefile b/Rakefile
index 438e67c..46dd4bd 100644
--- a/Rakefile
+++ b/Rakefile
@@ -39,7 +39,7 @@ namespace :build do
     mkdir_p(File.dirname(out_file), verbose: false)
     File.open(out_file, 'w') do |file|
       template = File.read('src/asciidoctor-interdoc-reftext.tmpl.js')
-      file << template.sub('//OPAL-GENERATED-CODE//', builder.to_s)
+      file << template.sub('//OPAL-GENERATED-CODE//') { builder.to_s }
     end
     File.binwrite "#{out_file}.map", builder.source_map
   end