Fix link in design_guidelines.md
Fix link in design_guidelines.md

Bug: none
Change-Id: Icec4837285930b30f8628242bcdd5e6018a0f72a
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2929192
Commit-Queue: Johan Bay <c4a91c97e1f8a218202ca733f9e9f5122f475e0f@google.com>
Reviewed-by: Mathias Bynens <17a2cbc4e33156874a9862b0f530521117e66a3d@chromium.org>

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend