Rename `Todo#to_reference` to `Todo#target_reference`

Since we're not actually returning a todo reference.

app/helpers/todos_helper.rb

@@ -16,7 +16,7 @@ module TodosHelper
 
   def todo_target_link(todo)
     target = todo.target_type.titleize.downcase
-    link_to "#{target} #{todo.to_reference}", todo_target_path(todo), { title: h(todo.target.title) }
+    link_to "#{target} #{todo.target_reference}", todo_target_path(todo), { title: todo.target.title }
   end
 
   def todo_target_path(todo)

app/models/todo.rb

@@ -71,7 +71,7 @@ class Todo < ActiveRecord::Base
     nil
   end
 
-  def to_reference
+  def target_reference
     if for_commit?
       target.short_id
     else

spec/models/todo_spec.rb

@@ -115,18 +115,18 @@ describe Todo, models: true do
     end
   end
 
-  describe '#to_reference' do
+  describe '#target_reference' do
     it 'returns the short commit id for commits' do
       subject.project = project
       subject.target_type = 'Commit'
       subject.commit_id = commit.id
 
-      expect(subject.to_reference).to eq commit.short_id
+      expect(subject.target_reference).to eq commit.short_id
     end
 
     it 'returns reference for issuables' do
       subject.target = issue
-      expect(subject.to_reference).to eq issue.to_reference
+      expect(subject.target_reference).to eq issue.to_reference
     end
   end
 end