Fix javadoc in StringEscapeUtils

Closes gh-6613

Author: jonatan-ivanov

micrometer-core/src/main/java/io/micrometer/core/instrument/util/StringEscapeUtils.java

@@ -31,9 +31,8 @@
  */
 public final class StringEscapeUtils {
 
-    /*
+    /**
      * @param v The string to escape.
-     *
      * @return An escaped JSON string.
      */
     public static String escapeJson(@Nullable String v) {