Fix misdirected link for TypingEnv

[steffahn]

The main thing that's fixed is not immediately visible in the diff, which is the link that uses that [tenv] alias in the first section:

When an environment to perform type system operations in has not yet been created, the TypingEnv can be used to bundle all of the external context required into a single type.
(quoting current state, TypingEnv leading to wrong place)