[docs] Use benchmark::IterationCount rather than int64_t in lambd…

…a to complexity (#940)

README.md

@@ -622,7 +622,7 @@ that might be used to customize high-order term calculation.
 
 ```c++
 BENCHMARK(BM_StringCompare)->RangeMultiplier(2)
-    ->Range(1<<10, 1<<18)->Complexity([](int64_t n)->double{return n; });
+    ->Range(1<<10, 1<<18)->Complexity([](benchmark::IterationCount n)->double{return n; });
 ```
 
 <a name="templated-benchmarks" />