Update grammar

From: Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>

src/backend/utils/mmgr/README

@@ -6,9 +6,9 @@ Notes About Memory Allocation Redesign
 Up through version 7.0, Postgres had serious problems with memory leakage
 during large queries that process a lot of pass-by-reference data.  There
 was no provision for recycling memory until end of query.  This needed to be
-fixed, even more so with the advent of TOAST which will allowed very large
-chunks of data to be passed around in the system.  This document describes
-the new memory management system implemented in 7.1.
+fixed, even more so with the advent of TOAST which allows very large chunks
+of data to be passed around in the system.  This document describes the new
+memory management system implemented in 7.1.
 
 
 Background