Update running-agents.mdx (#382)

Fix typo

Author: andrewboni

docs/src/content/docs/guides/running-agents.mdx

@@ -13,7 +13,7 @@ Agents do nothing by themselves – you **run** them with the `Runner` class or
 
 <Code lang="typescript" code={helloWorldExample} title="Simple run" />
 
-When you don't need a custom runner, you can also use the `run()` utility, which runs a singletone default `Runner` instance.
+When you don't need a custom runner, you can also use the `run()` utility, which runs a singleton default `Runner` instance.
 
 Alternatively, you can create your own runner instance: