Adjust abstract.

README.md

@@ -15,7 +15,7 @@ Sky Walking | [中文](https://github.com/wu-sheng/sky-walking/wiki/sky-walking-
 
 
 * Auto-instrumentation mechanism, **no need to CHANGE any application source code**. [Supported middlewares, frameworks and libraries](https://github.com/wu-sheng/sky-walking/wiki/3.0-supported-list).
-* Explicit-instrumentation, an [OpenTracing supported tracer](http://opentracing.io/documentation/pages/supported-tracers).
+* Explicit-instrumentation, as an [OpenTracing supported tracer](http://opentracing.io/documentation/pages/supported-tracers).
 * Pure Java server implementation, with RESTful Web service.
 * High performance streaming analysis.
 * The UI released on [wu-sheng/sky-walking-ui](https://github.com/wu-sheng/sky-walking-ui)