Course categories view /course/index.php, apply .generaltable class instead of .generalbox

Course categories view /course/index.php contains a table with classname generalbox. This gives rounded corners to the contents of the table (even though the table corners are not given the same border-radius). Looks strange and is an obscure enough page view that many theme designers won't catch it to override. Class should be generaltable instead.

Replicate:

1. Navigate to ../course/index.php?categoryedit=on
2. Observe table in center of page (note rounded corners on background color inside of square-cornered table)

Test:

1. Apply patch
2. Navigate to ../course/index.php?categoryedit=on
3. Observe table in center of page (note there are no longer rounded corners, table is styled according to .generaltable CSS)