fix a typo when deleting a row

[akkartik]

I believe struct erow requires the following invariant:

for (int i = 0; i < E.numrows; ++i)
  assert(E.row[i].idx == i);

This is important because erow.idx is used to index into E.row in various places. Incrementing idx can cause out of bounds access and memory corruption.

[akkartik]

Interestingly, the snaptoken tutorial for kilo includes this fix (in step 183). That gives me some confidence that I'm on the right track.

[StrongerXi]

I noticed it as well, and took me a while to convince myself until I saw the tutorial, and your PR here:).