MarkdownToHtml: Use String.equals to compare strings

As reported by SpotBugs:

  This code compares java.lang.String objects for reference equality
  using the == or != operators. Unless both strings are either constants
  in a source file, or have been interned using the String.intern()
  method, the same string value may be represented by two different
  String objects. Consider using the equals(Object) method instead.

Change-Id: Ie37a882900d02580f1f3d0d13327dfe9c45a746d

java/com/google/gitiles/doc/MarkdownToHtml.java

diff --git a/java/com/google/gitiles/doc/MarkdownToHtml.java b/java/com/google/gitiles/doc/MarkdownToHtml.java
index 49a90f0..2f0f0ff 100644
--- a/java/com/google/gitiles/doc/MarkdownToHtml.java
+++ b/java/com/google/gitiles/doc/MarkdownToHtml.java
@@ -242,7 +242,7 @@
           .close("span")
           .close("a");
       // github markdown compatibility
-      if (id != id.toLowerCase()) {
+      if (!id.equals(id.toLowerCase())) {
         html.open("a")
             .attribute("class", "h")
             .attribute("name", id.toLowerCase())