docs: fix ASAN instructions

No-Try: true
Docs-Preview: https://skia.org/?cl=166981
Change-Id: I80291d63fef1c5648109d0f03099a7e1ab64fbd4
Reviewed-on: https://skia-review.googlesource.com/c/166981
Auto-Submit: Hal Canary <halcanary@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
diff --git a/site/dev/testing/xsan.md b/site/dev/testing/xsan.md
index e3fa070..280015e 100644
--- a/site/dev/testing/xsan.md
+++ b/site/dev/testing/xsan.md
@@ -67,6 +67,7 @@
         cc = "${CLANGDIR}/bin/clang"
         cxx = "${CLANGDIR}/bin/clang++"
         sanitize = "ASAN"
+        extra_ldflags = [ "-fuse-ld=lld" ]
     EOF
     python tools/git-sync-deps
     bin/gn gen out/asan