Fixed typo in topics/http/sessions.txt.

For example, an attacker could log into ``good.example.com`` and get a valid

session for his account. If the attacker has control over ``bad.example.com``,

he can use it to send his session key to you since a subdomain is permitted

to set cookies on `*.example.com``. When you visit ``good.example.com``,

to set cookies on ``*.example.com``. When you visit ``good.example.com``,

you'll be logged in as the attacker and might inadvertently enter your

sensitive personal data (e.g. credit card info) into the attackers account.