(Hopefully) improve grammar

[CI skip]

test/mobilizon_web/resolvers/group_resolver_test.exs

@@ -106,7 +106,7 @@ defmodule MobilizonWeb.Resolvers.GroupResolverTest do
                group.preferred_username
     end
 
-    test "find_group/3 returns a group by it's username", context do
+    test "find_group/3 returns a group by its username", context do
       group = insert(:group)
 
       query = """

test/mobilizon_web/resolvers/participant_resolver_test.exs

@@ -141,7 +141,7 @@ defmodule MobilizonWeb.Resolvers.ParticipantResolverTest do
         |> post("/api", AbsintheHelpers.mutation_skeleton(mutation))
 
       assert hd(json_response(res, 200)["errors"])["message"] ==
-               "The event has already reached it's maximum capacity"
+               "The event has already reached its maximum capacity"
     end
 
     test "actor_join_event/3 should check the actor is owned by the user", %{

test/mobilizon_web/resolvers/person_resolver_test.exs

@@ -7,7 +7,7 @@ defmodule MobilizonWeb.Resolvers.PersonResolverTest do
   @non_existent_username "nonexistent"
 
   describe "Person Resolver" do
-    test "get_person/3 returns a person by it's username", context do
+    test "get_person/3 returns a person by its username", context do
       user = insert(:user)
       actor = insert(:actor, user: user)
 
@@ -44,7 +44,7 @@ defmodule MobilizonWeb.Resolvers.PersonResolverTest do
                "Person with ID 6895567 not found"
     end
 
-    test "find_person/3 returns a person by it's username", context do
+    test "find_person/3 returns a person by its username", context do
       user = insert(:user)
       actor = insert(:actor, user: user)
 

test/mobilizon_web/resolvers/user_resolver_test.exs

@@ -18,7 +18,7 @@ defmodule MobilizonWeb.Resolvers.UserResolverTest do
   @valid_single_actor_params %{preferred_username: "test2", keys: "yolo"}
 
   describe "Resolver: Get an user" do
-    test "find_user/3 returns an user by it's id", context do
+    test "find_user/3 returns an user by its id", context do
       user = insert(:user)
 
       query = """