Fix Checkstyle and rename test class to PowerOfFourTest

Author: Mansi5164

src/main/java/com/thealgorithms/bitmanipulation/PowerOfFour.java

@@ -5,9 +5,9 @@
  */
 public final class PowerOfFour {
 
-    // Private constructor to prevent instantiation
+    /** Private constructor to prevent instantiation. */
     private PowerOfFour() {
-        throw new AssertionError("Cannot instantiate utility class");
+        throw new AssertionError("Cannot instantiate utility class.");
     }
 
     /**

src/test/java/com/thealgorithms/bitmanipulation/PowerOfFourTest.java

@@ -4,7 +4,7 @@
 import org.junit.jupiter.api.Test;
 
 /**
- * Unit tests for PowerOfFour class.
+ * Unit tests for {@link PowerOfFour}.
  */
 public final class PowerOfFourTest {
 
@@ -30,4 +30,3 @@ void testNegativeNumbers() {
         Assertions.assertFalse(PowerOfFour.isPowerOfFour(-16));
     }
 }
-