docs: fix README health-check snippet's invalid aiohttp kwarg; widen add()/replace() type hints to accept dict shapes

the flagship health-check snippet called session.get(url, proxies=proxy) on a
plain aiohttp session; aiohttp's ClientSession.get/_request has no `proxies=`
kwarg (only singular `proxy:`), so the snippet TypeErrors immediately if
followed verbatim. pm.get()'s dict output is documented (aiohttp()'s own
docstring) as being for aioweb's ExtendedSession(proxies=...), so the snippet
now names that session type instead of a bare aiohttp one.

add()/replace() type hints omitted Dict[str, str] even though both already
call to_proxy() at runtime and accept dict-shaped proxy specs, same as the
burn()/restore()/is_burned()/remove() family. purely annotation, no behavior
change.

Signed-off-by: disqualifier <dev@disqualifier.me>

README.md

@@ -105,12 +105,14 @@ regardless of source.)
 
 ```python
 from aioproxies import AioProxies, ProxiesExhaustedError
+from aioweb import ExtendedSession
 
 pm = AioProxies(proxies=[...], cooldown=5)   # 5s reuse spacing; cooldown defaults to 0 (off)
 
 try:
     proxy = pm.get()                          # next usable proxy, aiohttp dict
-    resp = await session.get(url, proxies=proxy)
+    async with ExtendedSession(proxies=proxy) as session:
+        resp = await session.get(url)
     if response_looks_blocked(resp):
         pm.burn(proxy, 600)                   # time out 10 min ... or pm.burn(proxy) for dead
 except ProxiesExhaustedError: