Correct naming.

Author: KostyaSha

github-pullrequest-plugin/src/main/java/org/jenkinsci/plugins/github/pullrequest/GitHubPRLabel.java

@@ -47,9 +47,10 @@ public DescriptorImpl getDescriptor() {
         return (DescriptorImpl) Jenkins.getInstance().getDescriptor(GitHubPRLabel.class);
     }
 
-    @Symbol("githubLabels")
+    @Symbol("labels")
     @Extension
     public static class DescriptorImpl extends Descriptor<GitHubPRLabel> {
+        @Nonnull
         @Override
         public String getDisplayName() {
             return "Labels";

github-pullrequest-plugin/src/main/java/org/jenkinsci/plugins/github/pullrequest/events/impl/GitHubPRLabelExistsEvent.java

@@ -75,7 +75,7 @@ public boolean isSkip() {
         return skip;
     }
 
-    @Symbol("labelExists")
+    @Symbol("labelsExist")
     @Extension
     public static class DescriptorImpl extends GitHubPREventDescriptor {
         @Nonnull

github-pullrequest-plugin/src/main/java/org/jenkinsci/plugins/github/pullrequest/events/impl/GitHubPRLabelNotExistsEvent.java

@@ -78,7 +78,7 @@ public boolean isSkip() {
         return skip;
     }
 
-    @Symbol("labelNotExists")
+    @Symbol("labelsNotExist")
     @Extension
     public static class DescriptorImpl extends GitHubPREventDescriptor {
         @Nonnull

github-pullrequest-plugin/src/main/java/org/jenkinsci/plugins/github/pullrequest/events/impl/GitHubPRLabelPatternExistsEvent.java

@@ -64,7 +64,7 @@ public boolean isSkip() {
         return skip;
     }
 
-    @Symbol("labelPatternExists")
+    @Symbol("labelsPatternExists")
     @Extension
     public static class DescriptorImpl extends GitHubPREventDescriptor {
         @Nonnull

github-pullrequest-plugin/src/main/java/org/jenkinsci/plugins/github/pullrequest/events/impl/GitHubPRLabelRemovedEvent.java

@@ -98,7 +98,7 @@ public GitHubPRLabel getLabel() {
         return label;
     }
 
-    @Symbol("LabelsRemoved")
+    @Symbol("labelsRemoved")
     @Extension
     public static class DescriptorImpl extends GitHubPREventDescriptor {