fix: Param "scoreThreshold" should be "score_threshold" (#234)

Sine 18 Oct 2018, I found that the scoreThreshold param won't work and encounter  "Error: 3 INVALID_ARGUMENT: Request contains an invalid argument." during prediction call. I compared the doc below and the params should be score_threshold instead of scoreThreshold. 

https://cloud.google.com/vision/automl/docs/reference/rest/v1beta1/projects.locations.models/predict#request-body

samples/automl/automlVisionPredict.js

@@ -55,7 +55,7 @@ async function predict(
   const params = {};
 
   if (scoreThreshold) {
-    params.scoreThreshold = scoreThreshold;
+    params.score_threshold = scoreThreshold;
   }
 
   // Set the payload by giving the content and type of the file.