Fix typo (#3839)

Author: rupertw

dubbo-common/src/main/java/org/apache/dubbo/common/extension/SPI.java

@@ -27,7 +27,7 @@
  * Marker for extension interface
  * <p/>
  * Changes on extension configuration file <br/>
- * Use <code>Protocol</code> as an example, its configuration file 'META-INF/dubbo/com.xxx.Protocol' is changes from: <br/>
+ * Use <code>Protocol</code> as an example, its configuration file 'META-INF/dubbo/com.xxx.Protocol' is changed from: <br/>
  * <pre>
  *     com.foo.XxxProtocol
  *     com.foo.YyyProtocol