Fixed Softplus ref (#5755)

docs/ops/activation/SoftPlus_4.md

@@ -28,7 +28,7 @@ The `threshold` can be calculated with the following formula where `alpha` is th
 -log(e^{10^{-\alpha}} - 1.0) < threshold < log(\beta)
 \f]
 
-For example, if *T* is `fp32`, `threshold` should be `20` or if *T* is `fp16`, `threshold` should be `12`.
+For example, if *T* is `fp32`, `threshold` should be `20` or if *T* is `fp16`, `threshold` should be `11`.
 
 **Attributes**: *SoftPlus* operation has no attributes.
 

ngraph/core/reference/include/ngraph/runtime/reference/softplus.hpp

@@ -16,7 +16,7 @@ namespace ngraph
             template <typename T>
             void softplus(const T* arg, T* out, size_t count)
             {
-                const T threshold = static_cast<T>(-std::log(std::exp(std::pow(10, -6)) - 1));
+                const T threshold = static_cast<T>(std::log(std::numeric_limits<T>::max()));
 
                 for (size_t i = 0; i < count; i++)
                 {