docs: Change the klipper3d.org "language selection" icon

Change from the "material translate icon" to the "material language
icon" for the language selection icon.  (The "material language icon"
is an alias for the "material web icon" and it seems we can't use the
alias, so we directly set the icon to "material/web".)

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/_klipper3d/mkdocs.yml

@@ -49,6 +49,7 @@ theme:
   favicon: img/favicon.ico
   icon:
     repo: fontawesome/brands/github
+    alternate: material/web
   features:
       #- navigation.tabs
       #- navigation.expand