Simplify javanano command-line (#1298)

Tested: output is the same.
This was enabled by the change at 801e92007f

java/lib/protobuf-javanano-readme.md

@@ -14,7 +14,7 @@ Steps to update the protobuf library version:
   check in any changes to this file. The command from the root directory is:
 
   ```
-  protoc --javanano_out=com.google.i18n.phonenumbers.nano:java/libphonenumber/src \
+  protoc --javanano_out=java/libphonenumber/src \
          resources/phonemetadata.proto \
          --proto_path=resources
   ```