Link to "host bindings proposal" is broken.

[cldershem]

Describe the bug
The link in the Implementing Life section of the book to "host bindings proposal" is broken.

To Reproduce
Steps to reproduce the behavior:

1. Go to the book
2. Click on '"host bindings" proposal'
3. See error

Expected behavior
It is unclear what page is expected to load, possibly this explainer.

Screenshots
n/a

Additional context
It appears that "host bindings" has been renamed and that there have been some significant changes since the book was last updated. See issue #21 for more context.

[alexcrichton]

Thanks! I've updated the link and it should propagate to the website in a few minutes.