use api_find for UsersController#api_show

fixes CNVS-21714

test plan:
 * go to /api/v1/users/sis_user_id:theusersisid
 * it should not give a 404 (assuming you're logged in as an admin,
   and the id is valid)

Change-Id: I0901f5c60941c1b3d27aa3a699c3bed20b0850cd
Reviewed-on: https://gerrit.instructure.com/57746
Tested-by: Jenkins
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

app/controllers/users_controller.rb

@@ -1018,7 +1018,7 @@ class UsersController < ApplicationController
   #
   # @returns User
   def api_show
-    @user = params[:id] && params[:id] != 'self' ? User.find(params[:id]) : @current_user
+    @user = params[:id] && params[:id] != 'self' ? api_find(User, params[:id]) : @current_user
     if @user.grants_any_right?(@current_user, session, :manage, :manage_user_details)
       render :json => user_json(@user, @current_user, session, %w{locale avatar_url permissions}, @current_user.pseudonym.account)
     else