Apply explict RSpec/DescribedClass rubocop rule

We settled on using this style in commit 4cbe81a1, but didn't add the
rule enforcing this style and we didn't apply it to existing code.

spec/models/organization_spec.rb

@@ -48,24 +48,24 @@ describe Organization do
     let!(:organization) { create(:organization, name: "Watershed", user: create(:user, phone_number: "333")) }
 
     it "returns no results if search term is empty" do
-      expect(described_class.search(" ").size).to eq(0)
+      expect(Organization.search(" ").size).to eq(0)
     end
 
     it "finds fuzzily by name" do
-      expect(described_class.search("Greenpeace").size).to eq 0
-      search = described_class.search("Tershe")
+      expect(Organization.search("Greenpeace").size).to eq 0
+      search = Organization.search("Tershe")
       expect(search.size).to eq 1
       expect(search.first).to eq organization
     end
 
     scenario "finds by users email" do
-      search = described_class.search(organization.user.email)
+      search = Organization.search(organization.user.email)
       expect(search.size).to eq 1
       expect(search.first).to eq organization
     end
 
     scenario "finds by users phone number" do
-      search = described_class.search(organization.user.phone_number)
+      search = Organization.search(organization.user.phone_number)
       expect(search.size).to eq 1
       expect(search.first).to eq organization
     end