Spring boot example was missing the plugin to work correctly (#307)

Author: Macarse

graphql-kotlin-spring-example/pom.xml

@@ -37,6 +37,12 @@
         <sourceDirectory>${project.basedir}/src/main/kotlin</sourceDirectory>
         <testSourceDirectory>${project.basedir}/src/test/kotlin</testSourceDirectory>
         <plugins>
+            <plugin>
+                <groupId>org.springframework.boot</groupId>
+                <artifactId>spring-boot-maven-plugin</artifactId>
+                <version>${spring-boot.version}</version>
+            </plugin>
+
             <plugin>
                 <artifactId>kotlin-maven-plugin</artifactId>
                 <groupId>org.jetbrains.kotlin</groupId>