Update shell command to be more explicit

The command `solidus extension .` may not work directly on some machines
without `bundle exec`. Updated to be more explicit and prevent possible
confusion.

README.md

@@ -42,7 +42,7 @@ If you have an existing extension and want to update it to use the latest standa
 you can run the following in the extension's directory:
 
 ```console
-$ solidus extension .
+$ bundle exec solidus extension .
 ```
 
 In case of conflicting files, you will be prompted for an action. You can overwrite the files with