examples: upgrade protobuf-maven-plugin to 0.5.1

This was missed in 2094bb4

Author: ejona86

examples/pom.xml

@@ -76,7 +76,7 @@
       <plugin>
         <groupId>org.xolstice.maven.plugins</groupId>
         <artifactId>protobuf-maven-plugin</artifactId>
-        <version>0.5.0</version>
+        <version>0.5.1</version>
         <configuration>
           <protocArtifact>com.google.protobuf:protoc:3.5.1-1:exe:${os.detected.classifier}</protocArtifact>
           <pluginId>grpc-java</pluginId>