Fix missing backticks in errors

These weren't consistent. In some errors we were using backticks and
others we weren't. This fixes `connects_to` errors to use backticks
around the method name

activerecord/lib/active_record/connection_handling.rb

@@ -79,10 +79,10 @@ def establish_connection(config_or_env = nil)
     #
     # Returns an array of database connections.
     def connects_to(database: {}, shards: {})
-      raise NotImplementedError, "connects_to can only be called on ActiveRecord::Base or abstract classes" unless self == Base || abstract_class?
+      raise NotImplementedError, "`connects_to` can only be called on ActiveRecord::Base or abstract classes" unless self == Base || abstract_class?
 
       if database.present? && shards.present?
-        raise ArgumentError, "connects_to can only accept a `database` or `shards` argument, but not both arguments."
+        raise ArgumentError, "`connects_to` can only accept a `database` or `shards` argument, but not both arguments."
       end
 
       connections = []

activerecord/test/cases/base_test.rb

@@ -1666,7 +1666,7 @@ def test_protected_environments_are_stored_as_an_array_of_string
       Bird.connects_to(database: { writing: :arunit })
     end
 
-    assert_equal "connects_to can only be called on ActiveRecord::Base or abstract classes", error.message
+    assert_equal "`connects_to` can only be called on ActiveRecord::Base or abstract classes", error.message
   end
 
   test "cannot call connected_to on subclasses of ActiveRecord::Base" do

activerecord/test/cases/connection_adapters/connection_handlers_sharding_db_test.rb

@@ -233,7 +233,7 @@ def test_connects_to_raises_with_a_shard_and_database_key
           error = assert_raises(ArgumentError) do
             ActiveRecord::Base.connects_to(database: { writing: :arunit }, shards: { shard_one: { writing: :arunit } })
           end
-          assert_equal "connects_to can only accept a `database` or `shards` argument, but not both arguments.", error.message
+          assert_equal "`connects_to` can only accept a `database` or `shards` argument, but not both arguments.", error.message
         end
 
         def test_retrieve_connection_pool_with_invalid_shard