edit middleware docs code sample to use perf_counter as a timer

docs_src/middleware/tutorial001.py

@@ -7,8 +7,8 @@ app = FastAPI()
 
 @app.middleware("http")
 async def add_process_time_header(request: Request, call_next):
-    start_time = time.time()
+    start_time = time.perf_counter()
     response = await call_next(request)
-    process_time = time.time() - start_time
+    process_time = time.perf_counter() - start_time
     response.headers["X-Process-Time"] = str(process_time)
     return response