add jekyll-toc in group :jekyll_plugins

from https://jekyllrb.com/docs/continuous-integration/github-actions/

Author: norbertwg

Gemfile

@@ -16,6 +16,8 @@ gem "minima", "~> 2.5"
 # If you have any plugins, put them here!
 group :jekyll_plugins do
   gem "jekyll-feed", "~> 0.12"
+  # to create table of content for .md document
+  gem 'jekyll-toc'
 end
 
 # Windows and JRuby does not include zoneinfo files, so bundle the tzinfo-data gem
@@ -31,6 +33,4 @@ gem "wdm", "~> 0.1.1", :platforms => [:mingw, :x64_mingw, :mswin]
 
 gem "webrick", "~> 1.8"
 
-# to create table of content for .md document
-gem 'jekyll-toc'