Add lua-multi-incr.js example (#1692)

Also fix syntax error in the lua example in the README

Closes #1689.

examples/lua-multi-incr.js

@@ -0,0 +1,31 @@
+// Define a custome lua script that accepts two keys and an amount to
+// increment each of them by
+
+import { createClient, defineScript } from 'redis';
+
+async function luaMultiIncr() {
+  const client = createClient({
+    scripts: {
+      mincr: defineScript({
+        NUMBER_OF_KEYS: 2,
+        SCRIPT:
+          'return {' +
+          'redis.pcall("INCRBY", KEYS[1], ARGV[1]),' +
+          'redis.pcall("INCRBY", KEYS[2], ARGV[1])' +
+          '}',
+        transformArguments(key1, key2, increment) {
+          return [key1, key2, increment.toString()];
+        },
+      }),
+    },
+  });
+
+  await client.connect();
+
+  await client.set('mykey', '5');
+  console.log(await client.mincr('mykey', 'myotherkey', 10)); // [ 15, 10 ]
+
+  await client.quit();
+}
+
+luaMultiIncr();