Add padding character for bigtile mode to the symbol in the monster knowledge list

[backwardsEric]

Without it, the right half of the tile for a monster wasn't cleared on OS X for the rows beyond what's in the current category (i.e. going from a category with more monsters to one with fewer left clearing artifacts). I didn't see the same problem on Linux, but I didn't check if it was using bigtile mode.

[sil-quirk]

Issue was also replicable on Windows; it appears your fix solves it there also. Merging.