Fix rubocop warning

Author: toshimaru

Rakefile

@@ -3,29 +3,29 @@
 require 'yaml'
 require 'bundler'
 
-SOURCE_DIR = "src"
+SOURCE_DIR = 'src'
 INDEX_HTML = 'files/railties/RDOC_MAIN_rdoc.html'
 
-desc "Build railsdoc"
+desc 'Build railsdoc'
 task :build do
   generate_rails_rdoc
   generate_src
 
   sh 'bundle exec jekyll build'
 end
 
-desc "Switch to default Rails version"
+desc 'Switch to default Rails version'
 task :switch_default_rails do
-  switch_rails(config["default_rails_version"])
+  switch_rails(config['default_rails_version'])
 end
 
 desc "Build another version's railsdoc"
 task :build_multi do
-  config["rails_versions"].each do |version, detail|
+  config['rails_versions'].each do |version, detail|
     dir = "#{SOURCE_DIR}/#{version}"
     mkdir dir unless Dir.exist?(dir)
 
-    bulid_version = detail["specific_version"]
+    bulid_version = detail['specific_version']
     switch_rails(bulid_version)
     generate_rails_rdoc
     generate_src(target_version: version)
@@ -38,7 +38,7 @@ end
 
 def switch_rails(version)
   cd 'rails' do
-    sh "git reset --hard"
+    sh 'git reset --hard'
     sh "git switch refs/tags/v#{version} -C v#{version}"
   end
 end
@@ -47,9 +47,9 @@ def generate_rails_rdoc
   cd 'rails' do
     Bundler.with_unbundled_env do
       # replace sdoc gem
-      gemfile = File.read("Gemfile")
+      gemfile = File.read('Gemfile')
       gemfile.gsub!(/"sdoc.*$/, '"sdoc", github: "toshimaru/sdoc", branch: "railsdoc"')
-      File.write("Gemfile", gemfile)
+      File.write('Gemfile', gemfile)
 
       sh 'bundle install && bundle update sdoc'
       rm_rf 'doc'
@@ -59,7 +59,7 @@ def generate_rails_rdoc
 end
 
 def generate_src(target_version: nil)
-  copy_sources = Dir.glob('rails/doc/rdoc/*').reject { |path| path.end_with?("panel", "js", "created.rid") }
+  copy_sources = Dir.glob('rails/doc/rdoc/*').reject { |path| path.end_with?('panel', 'js', 'created.rid') }
   target_dir = "#{SOURCE_DIR}/#{target_version}"
   cp_r copy_sources, target_dir
 
@@ -69,8 +69,8 @@ def generate_src(target_version: nil)
     return if target_version.nil?
 
     # Replace absolute path in navigation.html
-    content = File.read("navigation.html")
-    content.gsub!("<a href=\"/", "<a href=\"/#{target_version}/")
-    File.write("navigation.html", content)
+    content = File.read('navigation.html')
+    content.gsub!('<a href=\"/', "<a href=\"/#{target_version}/")
+    File.write('navigation.html', content)
   end
 end