Fix cards display for preview pages by assigning to type 'about'.

Change-Id: Ia8433a6418ba54309ac2217621c70c7d81f4d611
diff --git a/src/com/google/doclava/DocFile.java b/src/com/google/doclava/DocFile.java
index c919d96..a0c88c0 100644
--- a/src/com/google/doclava/DocFile.java
+++ b/src/com/google/doclava/DocFile.java
@@ -390,7 +390,7 @@
         } else if (filename.indexOf("wear") == 0) {
           hdf.setValue("wear", "true");
         } else if (filename.indexOf("preview") == 0) {
-          hdf.setValue("page.type", "develop");
+          hdf.setValue("page.type", "about");
           hdf.setValue("page.category", "preview");
           hdf.setValue("preview", "true");
         } else if (filename.indexOf("auto") == 0) {