editconfig: set yml indent_size to 2

This is useful for github ci file

Author: beicause

.editorconfig

@@ -5,4 +5,7 @@ indent_style = space
 indent_size = 4
 end_of_line = lf
 charset = utf-8
-trim_trailing_whitespace = true
+trim_trailing_whitespace = true
+
+[*.y[a]ml]
+indent_size = 2