Fix optstring for showmap -f

This change modifies the optstring for showmap to specify that the -f
option requires an argument. Without this, 'showmap -f <filename>' will
always segfault.

Test: Confirm that this change fixes the '-f' option.
Bug: 243588957
Change-Id: Ibf75b1dc03d656dd2cc42b7c03c70588d8054d54
1 file changed