correct rubocop offenses

lib/github-pages-health-check/errors/deprecated_ip_error.rb

@@ -10,7 +10,7 @@ def message
           <<-MSG
             The custom domain for your GitHub Pages site is pointed at an outdated IP address.
             You must update your site's DNS records if you'd like it to be available via your custom domain.
-           MSG
+          MSG
         end
       end
     end

lib/github-pages-health-check/errors/invalid_a_record_error.rb

@@ -11,7 +11,7 @@ def message
              Your site's DNS settings are using a custom subdomain, #{domain.host},
              that's set up as an A record. We recommend you change this to a CNAME
              record pointing at #{username}.github.io.
-           MSG
+          MSG
         end
       end
     end

lib/github-pages-health-check/errors/invalid_aaaa_record_error.rb

@@ -11,7 +11,7 @@ def message
              Your site's DNS settings are using a custom subdomain, #{domain.host},
              that's set up with an AAAA record. GitHub Pages currently does not support
              IPv6.
-           MSG
+          MSG
         end
       end
     end

lib/github-pages-health-check/errors/invalid_cname_error.rb

@@ -11,7 +11,7 @@ def message
              Your site's DNS settings are using a custom subdomain, #{domain.host},
              that's not set up with a correct CNAME record.  We recommend you set this
              CNAME record to point at #{username}.github.io.
-           MSG
+          MSG
         end
       end
     end