Override EditorConfig settings for YAML files

I realized that I was fighting my editor, because we indent YAML files with
2 spaces, but `.editorconfig` specifies 4 spaces for all files.

Align the config with reality.

.editorconfig

@@ -15,3 +15,7 @@ end_of_line = lf
 charset = utf-8
 trim_trailing_whitespace = true
 insert_final_newline = true
+
+[*.{yml,yaml}]
+
+indent_size = 2