Update configure-a-non-voting-replica-set-member.txt

The use of letter 'an' is not grammatically correct here.

Author: askmrsinh

source/tutorial/configure-a-non-voting-replica-set-member.txt

@@ -38,7 +38,7 @@ set members to be non-voting members.
 
       cfg = rs.conf();
 
-   The returned document contains a ``members`` field which contains an
+   The returned document contains a ``members`` field which contains a
    zero-indexed array of member configuration documents, one document for each
    member of the replica set.