Change method name in doc to testDirectory

The old name `testTextFile` was unrelated to the code.

Author: stefanbirkner

README.md

@@ -90,7 +90,7 @@ If you need an empty directory then you can use the method
 `createDirectory(String)`.
 
     @Test
-    public void testTextFile() {
+    public void testDirectory() {
       sftpServer.createDirectory("/a/directory");
       //code that reads from or writes to that directory
     }

src/main/java/com/github/stefanbirkner/fakesftpserver/rule/FakeSftpServerRule.java

@@ -84,7 +84,7 @@
  * {@link #createDirectory(String)}.
  * <pre>
  * &#064;Test
- * public void testTextFile() {
+ * public void testDirectory() {
  *   sftpServer.{@link #createDirectory(String) createDirectory}("/a/directory");
  *   //code that reads from or writes to that directory
  * }