Fix: replace SyliusRefundPlugin by SyliusInvoicingPlugin in UPGRADE.md (#371)

Hi, I am not sure what the format is to contribute, I did not find
related documentation.

Sorry if it is not how it should be done, feel free to point out what I
should be doing.

Regarding this PR, it is a simple fix in the documentation UPGRADE.md
for Sylius 1.14.

Author: GSadee

UPGRADE.md

@@ -21,7 +21,7 @@
    The routes have been consolidated into a single `config/routes.yaml` file. It is sufficient to import this file instead of defining each route explicitly:
    ```yaml
        sylius_invoicing:
-           resource: "@SyliusRefundPlugin/config/routes.yaml"
+           resource: "@SyliusInvoicingPlugin/config/routes.yaml"
    ```
    
    However, if you have customized the routes, you need to adjust the paths to reflect the new structure: