Don't return implementation from ElasticApmTracer

This confuses IDEs as ApmSpanBuilder is package private

Author: felixbarny

apm-opentracing/src/main/java/co/elastic/apm/opentracing/ElasticApmTracer.java

@@ -49,7 +49,7 @@ public ApmSpan activeSpan() {
     }
 
     @Override
-    public ApmSpanBuilder buildSpan(String operationName) {
+    public SpanBuilder buildSpan(String operationName) {
         return new ApmSpanBuilder(operationName, scopeManager());
     }