📝 Add missing compresslevel parameter on docs

The compresslevel parameter was added in encode/starlette#1128 encode/starlette#2553
But, it was not added to the document, so an explanation was added

docs/en/docs/advanced/middleware.md

@@ -85,6 +85,7 @@ The middleware will handle both standard and streaming responses.
 The following arguments are supported:
 
 * `minimum_size` - Do not GZip responses that are smaller than this minimum size in bytes. Defaults to `500`.
+* `compresslevel` - Used during GZip compression. It is an integer ranging from 1 to 9. Defaults to `9`. Lower value results in faster compression but larger file sizes, while higher value results in slower compression but smaller file sizes.
 
 ## Other middlewares
 

docs_src/advanced_middleware/tutorial003.py

@@ -3,7 +3,7 @@
 
 app = FastAPI()
 
-app.add_middleware(GZipMiddleware, minimum_size=1000)
+app.add_middleware(GZipMiddleware, minimum_size=1000, compresslevel=9)
 
 
 @app.get("/")