Moving CrailFS to CrailStore

In the source code, CrailFS is renamed to CrailStore.
diff --git a/site/overview/index.md b/site/overview/index.md
index 085d5df..5db1e84 100644
--- a/site/overview/index.md
+++ b/site/overview/index.md
@@ -77,10 +77,10 @@
 </p>
 </div>
     CrailConfiguration conf = new CrailConfiguration();
-    CrailFS fs = CrailFS.newInstance(conf);
-    CrailFile file = fs.createFile(filename, 0, 0).get().syncDir();
+    CrailStore store = CrailStore.newInstance(conf);
+    CrailFile file = store.createFile(filename, 0, 0).get().syncDir();
     CrailOutputStream outstream = file.getDirectOutputStream();
-    ByteBuffer dataBuf = fs.allocateBuffer();
+    ByteBuffer dataBuf = store.allocateBuffer();
     Future<DataResult> future = outputStream.write(dataBuf);
     ...
     future.get();