Update src/main/java/com/jakewharton/apibuilder/ApiBuilder.java

placeHolderBuilder shouldn't contain the delimiters "{" and "}" because the key in the this.fieldsMap never contain delimiters mentioned above.
as to the key in the this.fieldsMap you can find them in the all kinds of pingDom services class in package com.jakewharton.pingdom.services;