Fix RSpec/DescribeSymbol cop violations

spec/lib/gitlab/git/blob_snippet_spec.rb

spec/lib/gitlab/git/blob_spec.rb

spec/lib/gitlab/git/commit_spec.rb

spec/lib/gitlab/git/compare_spec.rb

spec/lib/gitlab/git/diff_collection_spec.rb

spec/lib/gitlab/git/tree_spec.rb

spec/lib/gitlab/git/util_spec.rb

spec/lib/gitlab/ldap/user_spec.rb

spec/models/milestone_spec.rb

spec/models/namespace_spec.rb

spec/models/pages_domain_spec.rb

spec/models/repository_spec.rb

spec/requests/api/projects_spec.rb

spec/requests/api/v3/projects_spec.rb

spec/services/milestones/close_service_spec.rb

spec/services/projects/fork_service_spec.rb