Document missing code to run Gradle bootRun task

See gh-25012

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/asciidoc/running.adoc

@@ -112,7 +112,7 @@ Gradle allows project properties to be set in a variety of ways, including on th
 
 [source,bash,indent=0,subs="verbatim,attributes"]
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-$ ./gradlew -Pexample=custom
+$ ./gradlew bootRun -Pexample=custom
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 The preceding example sets the value of the `example` project property to `custom`.