Docs: fix small typos in Extensions doc (#3138)

Co-authored-by: Tom Christie <tom@tomchristie.com>

docs/advanced/extensions.md

@@ -2,7 +2,7 @@
 
 Request and response extensions provide a untyped space where additional information may be added.
 
-Extensions should be used for features that may not be available on all transports, and that do not fit neatly into [the simplified request/response model](https://www.encode.io/httpcore/extensions/) that the underlying `httpcore` pacakge uses as it's API.
+Extensions should be used for features that may not be available on all transports, and that do not fit neatly into [the simplified request/response model](https://www.encode.io/httpcore/extensions/) that the underlying `httpcore` package uses as its API.
 
 Several extensions are supported on the request:
 
@@ -239,4 +239,4 @@ with httpx.stream("GET", "https://www.example.com") as response:
 
     ssl_object = network_stream.get_extra_info("ssl_object")
     print("TLS version", ssl_object.version())
-```
+```