Fix heading of CPU-speed design

This document really only talks about CPU speed.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>

doc/design-cpu-speed.rst

-===============================================
-Taking relative CPU and disk speed into account
-===============================================
+======================================
+Taking relative CPU speed into account
+======================================
 
 .. contents:: :depth: 4