website: Update minimum Clang version to 6

Due to changes here:
https://gem5-review.googlesource.com/c/public/gem5/+/45621,
the minimum supported Clang version is now 6.

Change-Id: I81c5215c3696225990ecb0cf185aec93d9c75c35
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/45699
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>

_pages/documentation/general_docs/building/index.md

diff --git a/_pages/documentation/general_docs/building/index.md b/_pages/documentation/general_docs/building/index.md
index 1c29599..d114967 100644
--- a/_pages/documentation/general_docs/building/index.md
+++ b/_pages/documentation/general_docs/building/index.md
@@ -39,7 +39,7 @@
 * **git** : gem5 uses git for version control.
 * **gcc**: gcc is used to compiled gem5. **Version >=5 must be used**. We
 support up to gcc Version 10.
-* **Clang**: Clang can also be used. At present, we support Clang 3.9 to
+* **Clang**: Clang can also be used. At present, we support Clang 6 to
 Clang 9 (inclusive).
 * **SCons** : gem5 uses SCons as its build environment. SCons 3.0 or greater
 must be used.