Fixed version for gradle in java Readme.md

Looks like this was forgotten by the last time this was edited.

java/README.md

@@ -45,7 +45,7 @@ protobuf-java-util package:
 
 If you are using Gradle, add the following to your `build.gradle` file's dependencies:
 ```
-    implementation 'com.google.protobuf:protobuf-java:3.11.0'
+    implementation 'com.google.protobuf:protobuf-java:3.15.3'
 ```
 Again, be sure to check that the version number matches (or is newer than) the version number of protoc that you are using.