index.rst: fix gf pdk variant #1379

Job	Run time
download_caravel	1m 31s
download_deps (pdk-with-volare)	3m 8s
download_deps (install_mcw)	4m 27s
download_deps (openlane)	51s
download_deps (setup-timing-scripts)	38s
download_deps (precheck)	1m 21s
RTL-verification (sky130A)	4m 11s
hardening (sky130A)	56m 33s
hardening (sky130B)	54m 55s
RTL-verification (sky130B)	3m 58s
STA (sky130A)	8m 43s
precheck (sky130A)	35m 50s
precheck (sky130B)	37m 14s
	3h 33m 20s

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