Fix yaml test name

Author: JonasKunz

x-pack/plugin/mapper-exponential-histogram/src/yamlRestTest/resources/rest-api-spec/test/10_synthetic_source.yml

@@ -154,7 +154,7 @@ setup:
         scale: -7
 
 ---
-"Empty Zero Bucket but with count":
+"Empty Zero Bucket but with custom threshold":
   - do:
       index:
         index: test_exponential_histogram