docs: Fix typo in plugins documentation (#4145)

docs/docs/guides/developer-guide/plugins/index.mdx

@@ -7,7 +7,7 @@ import Tabs from '@theme/Tabs';
 import TabItem from '@theme/TabItem';
 
 The heart of Vendure is its plugin system. Plugins not only allow you to instantly add new functionality to your
-Vendure server via third-part npm packages, they are also the means by which you build out the custom business
+Vendure server via third-party npm packages, they are also the means by which you build out the custom business
 logic of your application.
 
 Plugins in Vendure allow one to: