changed normal sampler standard deviation json key to "stddev"

com.unity.perception/Runtime/Randomization/Scenarios/Serialization/ScenarioSerializer.cs

                     min = normalSampler.range.minimum,
                     max = normalSampler.range.maximum,
                     mean = normalSampler.mean,
-                    standardDeviation = normalSampler.standardDeviation
+                    stddev = normalSampler.standardDeviation
                 };
             else
                 throw new ArgumentException($"Invalid sampler type ({sampler.GetType()})");
                         maximum = (float)normalSampler.max
                     },
                     mean = (float)normalSampler.mean,
-                    standardDeviation = (float)normalSampler.standardDeviation
+                    standardDeviation = (float)normalSampler.stddev
                 };
             throw new ArgumentException($"Cannot deserialize unsupported sampler type {samplerOption.GetType()}");
         }

com.unity.perception/Runtime/Randomization/Scenarios/Serialization/SerializationStructures.cs

         public double min;
         public double max;
         public double mean;
-        public double standardDeviation;
+        public double stddev;
     }
 
     class ConstantSampler : ISamplerOption