SLEPc Installation Instructions

[UZerbinati]

Dear all,
I think there is a typo in the README instruction for SLEPc.
I think the configuration options for SLEPc should be either --download-blopex or --with-blopex=1.

[joseeroman]

I think this is not very important, but you can create a Pull Request for this change if you want. The --download-blopex is usually more convenient as it uses the same compiler and flags to build both SLEPc and BLOPEX.