Updated chapter names in Arch guide

Chapter names were mixed up:

Multi-site chapter was wrongly named as Hybrid
Hybrid chapter was titled as General Purpose

Change-Id: I8c8062c975ed26687d9b5e5fb5c72f4c1a5f4e3b

doc/arch-design/ch_hybrid.xml

@@ -4,7 +4,7 @@
   xmlns:xlink="http://www.w3.org/1999/xlink"
   version="5.0"
   xml:id="hybrid">
-    <title>General purpose</title>
+    <title>Hybrid</title>
 
     <xi:include href="hybrid/section_introduction_hybrid.xml"/>
     <xi:include href="hybrid/section_user_requirements_hybrid.xml"/>

doc/arch-design/ch_multi_site.xml

@@ -4,7 +4,7 @@
   xmlns:xlink="http://www.w3.org/1999/xlink"
   version="5.0"
   xml:id="multi_site">
-    <title>Hybrid</title>
+    <title>Multi-site</title>
 
     <xi:include href="multi_site/section_introduction_multi_site.xml"/>
     <xi:include href="multi_site/section_user_requirements_multi_site.xml"/>