Markdown: fix SELF_CLOSING_TAGS.contains call

CID 29319 (#1 of 1): EC: Comparing incompatible types for equality (FB.EC_UNRELATED_TYPES)
1. defect: Call to com.google.common.collect.ImmutableSet<java.lang.String>.equals(String).

This should be .contains() to test the argument is a member of the
set.  By mistake I wrote .equals(), which is always false as a set
is never equal to a string.

Change-Id: Ic1908c47a37e6ad930f182aac4ce0a29f86452b1

gitiles-servlet/src/main/java/com/google/gitiles/doc/html/HtmlBuilder.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/doc/html/HtmlBuilder.java b/gitiles-servlet/src/main/java/com/google/gitiles/doc/html/HtmlBuilder.java
index 6676ed3..093f901 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/doc/html/HtmlBuilder.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/doc/html/HtmlBuilder.java
@@ -142,7 +142,8 @@
 
   /** Close an open tag with {@code </tag>} */
   public HtmlBuilder close(String tag) {
-    checkArgument(ALLOWED_TAGS.contains(tag) && !SELF_CLOSING_TAGS.equals(tag),
+    checkArgument(
+        ALLOWED_TAGS.contains(tag) && !SELF_CLOSING_TAGS.contains(tag),
         "invalid HTML tag %s", tag);
 
     finishActiveTag();