Rephrase readme so that it doesn't seem to suggest that PESI can make response times worse.

README.rst

@@ -61,7 +61,7 @@ for B and C.
 
 To summarize:
 
-* Parallel ESI can *substantially* increase the response times for ESI
+* Parallel ESI can *substantially* improve the response times for ESI
   if cacheable objects include many uncacheable objects. The maximum
   benefit, compared with standard, serial processing, is achieved
   through parallel ESI in cases where all nodes of an ESI tree are