Fix typo in signalEntity client sample function

Resolves https://github.com/MicrosoftDocs/azure-docs/issues/44739.

Author: cgillum

articles/azure-functions/durable/durable-functions-entities.md

@@ -182,7 +182,7 @@ const df = require("durable-functions");
 module.exports = async function (context) {
     const client = df.getClient(context);
     const entityId = new df.EntityId("Counter", "myCounter");
-    await context.df.signalEntity(entityId, "add", 1);
+    await client.signalEntity(entityId, "add", 1);
 };
 ```