Added alt attribute in image map input problem.

Tickets: [TNL-822] (https://openedx.atlassian.net/browse/TNL-822), [TNL-825] (https://openedx.atlassian.net/browse/TNL-825)

Without ALT attribute image map input problem type is completely inaccessible. It's important that we include one in the example to demonstrate it to course team.

Space removed from ```for``` attribute.