romdisk: fix a bug in the "hexdump" command that printed an uninitial…

…ized character

After an ldir, DE points to the next character to set and NOT HL...
This resulted in the last character of the buffer being uninitialized, yet
printed at the end of the function.

romdisk/hexdump.asm

@@ -147,7 +147,8 @@ hexdump_print_bc_chars:
     ldir
     pop bc
     ; Store a newline at the end of the buffer
-    ld (hl), '\n'
+    ld a, '\n'
+    ld (de), a
     ; Fill the buffer with the actual line to print
     ; Start by getting the current offset in the file, convert it to ASCII
     ld hl, (file_offset)