Fix typo in conventions.ipynb

[jjgomezcadenas]

In notebook:

$f$ refers to an object, and $f_j$ refers to the value of the object at voxel $j$

• $g$ refers to a set of projections, and $g_i$ refers to the value of the projections at detector element $i$
• $H$ refers to the system matrix with components $H_{ij}$: the contribution voxel $j$ in object space makes to detector element $j$ in image space

Should be

to detector element $i$ in image space. (NB: "i" instead of "j"