Fix typo in README.md (#16)

diff --git a/README.md b/README.md
index f6ca3e9..b600d78 100644
--- a/README.md
+++ b/README.md
@@ -83,7 +83,7 @@
     if (counter != measurements.cpu->end())
     {
         // Get the data stored in the counter, casted to the type you need
-        auto value = cpu_res->counter.get<float>();
+        auto value = counter->second.get<float>();
     }
 }
 ```