Fix HTML error in themanage_proj_edit_page.php

Fix HTML issue reported by Nu Html Checker:

• Error: Start tag fieldset seen in table.
• Error: Table column 5 established by element th has no cells
  beginning in it
• Error: " in an unquoted attribute value.
• Warning: A table row was 7 columns wide, which is less than the column
  count established by the first row (8).

Center table header cells on corresponding data cells.