README.md: Update reference to xc7
README.md: Update reference to xc7

The documentation of the "Xilinx Series 7" was relocated in commit:
a1ab7beb03c8f7ef6756b94fddc4e0d947832a67

Adjust the reference accordingly.

Signed-off-by: Marcello Sylvester Bauer <59f50e73a96730abf0e1fcdd68d04e0ab4294f57@sylv.io>

SymbiFlow/symbiflow-arch-defs,SymbiFlow/symbiflow-arch-defs