doc/stdenv: fix typo in dependency propagation

dep(h0, t0, A, B)

propagated-dep(h1, t1, B, C)

h0 + h1 in {-1, 0, 1}

h0 + t1 in {-1, 0, -1}

h0 + t1 in {-1, 0, 1}

----------------------------- Take immediate dependencies' propagated dependencies

propagated-dep(mapOffset(h0, t0, h1),

               mapOffset(h0, t0, t1),

dep(h, t, A, B)

```

Some explanation of this monstrosity is in order. In the common case, the target offset of a dependency is the successor to the target offset: `t = h + 1`. That means that:

Some explanation of this monstrosity is in order. In the common case, the target offset of a dependency is the successor to the host offset: `t = h + 1`. That means that:

```

let f(h, t, i) = i + (if i <= 0 then h else t - 1)