[Rd] Minor error in area() documentation (PR#3378)

[arnima at u.washington.edu]

Dear r-bugs,

The 'see also' link in help(area,html=T) is broken, see line 91 in
library/MASS/html/area.html:

"../../integrate/html/integrate.html"
should be
"../../base/html/integrate.html"

Regards,
Arni

OS:   Windows XP
R:    1.7.1
MASS: 7.1-8