Fix typo in SessionMiddleware docs (#2559)

docs/middleware.md

@@ -106,7 +106,7 @@ The following arguments are supported:
 * `same_site` - SameSite flag prevents the browser from sending session cookie along with cross-site requests. Defaults to `'lax'`.
 * `path` - The path set for the session cookie. Defaults to `'/'`.
 * `https_only` - Indicate that Secure flag should be set (can be used with HTTPS only). Defaults to `False`.
-* `domain` - Domain of the cookie used to share cookie between subdomains or cross-domains. The browser defaults the domain to the same host that set the cookie, excluding subdomains [refrence](https://developer.mozilla.org/en-US/docs/Web/HTTP/Cookies#domain_attribute).
+* `domain` - Domain of the cookie used to share cookie between subdomains or cross-domains. The browser defaults the domain to the same host that set the cookie, excluding subdomains ([reference](https://developer.mozilla.org/en-US/docs/Web/HTTP/Cookies#domain_attribute)).
 
 
 ```python