Fix sentence in clustered reduction description (#690)

Author: svenvh

ext/cl_khr_subgroup_extensions.asciidoc

@@ -638,7 +638,7 @@ This extension adds support for clustered reductions that operate on a subset of
 
 ==== Add a new Section 6.15.X - Subgroup Clustered Reduction Built-in Functions
 
-This section describes arithmetic operations that are performed subset of work items in a subgroup, referred to as a cluster.
+This section describes arithmetic operations that are performed on a subset of work items in a subgroup, referred to as a cluster.
 A cluster is described by a specified cluster size.
 Work items in a subgroup are assigned to clusters such that for cluster size _n_, the _n_ work items in the subgroup with the smallest subgroup local IDs are assigned to the first cluster, then the _n_ remaining work items with the smallest subgroup local IDs are assigned to the next cluster, and so on.
 The specified cluster size must be an integer constant expression that is a power-of-two.