Update link to pg-sql2 (#272)

Was linking to the old repo

Author: rattrayalex

src/pages/postgraphile/make-extend-schema-plugin.md

@@ -294,7 +294,7 @@ const matchingUserResolver = async (parent, args, context, resolveInfo) => {
 
 `queryBuilder` is an instance of `QueryBuilder`, a helper that uses an SQL AST
 constructed via
-[`pg-sql2` methods](https://github.com/graphile/pg-sql2/blob/master/README.md)
+[`pg-sql2` methods](https://www.npmjs.com/package/pg-sql2#api)
 to dynamically create powerful SQL queries without risking SQL injection
 attacks. The `queryBuilder` has a number of methods which affect the query which
 will be generated. The main ones you're likely to want are: