Add header anchors to common device page sections.

Change-Id: I5b9a88a3c973a95725095e825db9fa8cb2b342ae
diff --git a/_includes/appinstall.html b/_includes/appinstall.html
index cce8a76..5066ebb 100644
--- a/_includes/appinstall.html
+++ b/_includes/appinstall.html
@@ -1,4 +1,4 @@
-<div class='page-heading'>App Install Method (Requires Root):</div>
+<div class='page-heading' id='app-install'>App Install Method (Requires Root):</div>
 <hr />
 <p class="text">Download the TWRP&nbsp;Manager app:</p>
 <ul class="text">