Fix checkstyle: trailing spaces and missing newline at end of files

Author: Rosander0

src/main/java/com/thealgorithms/strings/LongestCommonSubstring.java

@@ -4,7 +4,7 @@
  * Longest Common Substring finds the longest string that is a
  * contiguous substring of two input strings.
  * Example: "abcdef" and "zcdemf" -> "cde"
- * 
+ *
  * author: Vraj Prajapati @Rosander0
  */
 public final class LongestCommonSubstring {
@@ -44,4 +44,4 @@ public static String longestCommonSubstring(final String a, final String b) {
         }
         return a.substring(endIndex - maxLength, endIndex);
     }
-}
+}

src/test/java/com/thealgorithms/strings/LongestCommonSubstringTest.java

@@ -31,4 +31,4 @@ public void testMultipleMatchesFirstLongest() {
         // Keeps the first matched longest substring when lengths are tied
         assertEquals("abc", LongestCommonSubstring.longestCommonSubstring("abcXdef", "abcYdef"));
     }
-}
+}