Fix wrong document about Force Legacy Completions Endpoint (#8133)

Author: letanloc1998

docs/customize/model-providers/top-level/openai.mdx

@@ -90,7 +90,7 @@ If you are using an OpenAI API compatible providers, you can change the `apiBase
 
 ### How to Force Legacy Completions Endpoint Usage
 
-To force usage of `chat/completions` instead of `completions` endpoint you can set:
+To force usage of `completions` instead of `chat/completions` endpoint you can set:
 
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