Fix Javadoc warnings

Change-Id: Ia5fe7785f76da92abbf940599cbd3d7dc6ac2cc5
diff --git a/core/api/src/main/java/org/onosproject/net/intent/constraint/ProtectionConstraint.java b/core/api/src/main/java/org/onosproject/net/intent/constraint/ProtectionConstraint.java
index a337040..e4dce01 100644
--- a/core/api/src/main/java/org/onosproject/net/intent/constraint/ProtectionConstraint.java
+++ b/core/api/src/main/java/org/onosproject/net/intent/constraint/ProtectionConstraint.java
@@ -61,7 +61,7 @@
     /**
      * Returns protection constraint.
      *
-     * @return
+     * @return protection constraint
      */
     public static ProtectionConstraint protection() {
         return PROTECTION_CONSTRAINT;