docs/ssl: fix typo (#3703)

Author: xrmx

docs/advanced/ssl.md

@@ -29,7 +29,7 @@ import certifi
 import httpx
 import ssl
 
-# This SSL context is equivelent to the default `verify=True`.
+# This SSL context is equivalent to the default `verify=True`.
 ctx = ssl.create_default_context(cafile=certifi.where())
 client = httpx.Client(verify=ctx)
 ```