-b, --build-depends should specify that it can take more than one argument

[emilypi]

-b, --build-depends supports multiple arguments as of #5845, but the help docstring fails to mention the correct syntax. The current help doc is:

 -b --build-depends=DEPENDENCY                           Include an additional package
                                                         in the environment presented
                                                         to GHCi.

This should be changed to reflect a potential plurality of arguments, DEPENDENCIES.