Fix mistake in README

Author: Roman Kennke

README.md

@@ -1,6 +1,6 @@
 # Shenandoah Visualizer
 
-Shenandoah Visualizer is the low-level tool to visualize the internal state of
+Shenandoah Visualizer is a low-level tool to visualize the internal state of
 [Shenandoah GC](https://wiki.openjdk.java.net/display/Shenandoah). It relies on
 jvmstat interface to pull the data from the live JVM.