Add `by` keyword to start the proofs

[therewillbecode]

Add the by keyword to two proofs where it seemed to be missing. Without these the student might be confused by errors messages in their editor that don't make it obvious that this action is needed.

[hrmacbeth]

Thanks! This is a good idea. I'll do it consistently throughout in the next revision (I think it needs doing all through Chapter 1).

(The point is that if I intended a proof to be a calc-only proof, the by is optional, but someone trying a different approach will need it.)

[therewillbecode]

Thanks! This is a good idea. I'll do it consistently throughout in the next revision (I think it needs doing all through Chapter 1).

(The point is that if I intended a proof to be a calc-only proof, the by is optional, but someone trying a different approach will need it.)

Yeah that makes sense. In VSCode the goal doesn't appear in the info view without writing calc or by which is what I found a bit confusing initially.