docs: fixup indentation

The most canonical indentation-style here is two spaces, which is what
the standard boilerplate in all documents use. So let's normalize to
that.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3443>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3443>
diff --git a/.editorconfig b/.editorconfig
index db1f4a9..3f9e475 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -32,6 +32,10 @@
 indent_style = space
 indent_size = 2
 
+[*.html]
+indent_style = space
+indent_size = 2
+
 [*.patch]
 trim_trailing_whitespace = false