Clarify looping over AsyncSequence in file reading example

Author: swifty-amay

docs/advanced/files.md

@@ -15,7 +15,11 @@ let readComplete: EventLoopFuture<Void> = req.fileio.readFile(at: "/path/to/file
 // Or
 
 let file = try await request.fileio.readFile(at: path, chunkSize: 16 * 1024) // 32Kb, ~5 chunks
-    
+for try await chunk in file {
+    readContent += String(buffer: chunk)                    //converting chunks into string
+}  
+print(readContent) 
+ 
 // Read is complete
 ```