bmcweb: Fix content type for javascript files

Somehow we got the content type wrong on javascript files, which became
apparent when the content security patches were coming out.

This patchset corrects the string.

Tested By:
Opened webui in chrome, and inspected the javascript files.  content
type was correct.

Change-Id: I9ba706d5b919bf4c6365d01853c978b1616913d4
Signed-off-by: Ed Tanous <ed.tanous@intel.com>