Update user_guide.md (#1296)

docs/user_guide.md

diff --git a/docs/user_guide.md b/docs/user_guide.md
index dff6d03..34bea69 100644
--- a/docs/user_guide.md
+++ b/docs/user_guide.md
@@ -760,13 +760,13 @@
 
 ```c++
 static void BM_MultiThreaded(benchmark::State& state) {
-  if (state.thread_index == 0) {
+  if (state.thread_index() == 0) {
     // Setup code here.
   }
   for (auto _ : state) {
     // Run the test as normal.
   }
-  if (state.thread_index == 0) {
+  if (state.thread_index() == 0) {
     // Teardown code here.
   }
 }