website: Remove incorrect ISA 'ARCH' in building docs

Change-Id: I4d7b1ec074ff0a8d20d9feee31b1b5ed8540ffb3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/55983
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
diff --git a/_pages/documentation/general_docs/building/index.md b/_pages/documentation/general_docs/building/index.md
index f07ccdc..a2af8e8 100644
--- a/_pages/documentation/general_docs/building/index.md
+++ b/_pages/documentation/general_docs/building/index.md
@@ -150,7 +150,6 @@
 
 The valid ISAs are:
 
-* ARCH
 * ARM
 * NULL
 * MIPS