Fix paths on the download page

[Ingo Blechschmidt]

Dear FEniCS web team,

there appear to be three small problems with the download page at https://fenicsproject.org/download/:

1. The automated build script has a size of zero bytes.

2. The git clone commands don't work. The SSH addresses have to be changed for the HTTPS addresses, for instance https://bitbucket.org/fenics-project/ufl instead of [e-mail]:fenics-project/ufl.

3. All of the cd/pip3 lines have to be enclosed in parenthesis, so that the effect of changing the working directory is only temporary.

Thank you for your efforts. :-)