add Bessel's correction to stdev calculation

Co-authored-by: Johannes Gäßler <johannesg@5d6.de>

examples/llama-bench/llama-bench.cpp

@@ -63,7 +63,7 @@ T stddev(const std::vector<T>& v) {
     }
     T mean = avg(v);
     T sq_sum = std::inner_product(v.begin(), v.end(), v.begin(), T(0));
-    T stdev = std::sqrt(sq_sum / (T)v.size() - mean * mean);
+    T stdev = std::sqrt(sq_sum / (T)(v.size() - 1) - mean * mean * (T)v.size() / (T)(v.size() - 1));
     return stdev;
 }