Fix the italics and fixed width text being nested wrong.

[Rendered](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/15cf6e15f257df224282517c4d79a2308e811313/Documentation/MemoryManagement.md)

[Old version](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/bcb3d9a680adcea12927a74dea9216b0e40d96b5/Documentation/MemoryManagement.md)

Note that I had to add a space between `@unsafeify` and the following comma, so it line-breaks wrong now.

I'm not sure whether this is to be considered a bug in how Github renders overlapping tags, given how the flexibilities markdown implementations take with the "spec", and how the "spec" isn't really a formal specification to begin with, but just for future reference, where does one report bugs with Github?