docs: fix for rubocop's Style/StringConcatenation when uncomment it

Author: nishidayuya

Gemfile

@@ -3,7 +3,7 @@
 source "https://rubygems.org"
 
 # Uncomment this to use local copy of simplecov-html in development when checked out
-# gem "simplecov-html", path: File.dirname(__FILE__) + "/../simplecov-html"
+# gem "simplecov-html", path: File.join(__dir__, "../simplecov-html")
 
 # Uncomment this to use development version of html formatter from github
 # gem "simplecov-html", github: "simplecov-ruby/simplecov-html"