uaiohttpclient: Fix missing name in unreachable example code.

As-written this code is unreachable (return statement two line above),
so this change is really just to make the linter happy.

Found by Ruff checking F821.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

micropython/uaiohttpclient/example.py

@@ -9,7 +9,7 @@ def print_stream(resp):
     print((yield from resp.read()))
     return
     while True:
-        line = yield from reader.readline()
+        line = yield from resp.readline()
         if not line:
             break
         print(line.rstrip())