Fix struct syntax regex

Author: sodapopcan

Diff for: syntax/elixir.vim

@@ -72,7 +72,7 @@ syn match elixirListDelimiter '\[' contained containedin=elixirList
 syn region elixirList matchgroup=elixirListDelimiter start='\[' end='\]' contains=ALLBUT,@elixirNotTop fold
 
 syn match elixirStructDelimiter '{' contained containedin=elixirStruct
-syn region elixirStruct matchgroup=elixirStructDelimiter start="%\(\w\+{\)\@=" end="}" contains=ALLBUT,@elixirNotTop fold
+syn region elixirStruct matchgroup=elixirStructDelimiter start="%\%([A-Z][A-Za-z.]\+{\)\@=" end="}" contains=ALLBUT,@elixirNotTop fold
 
 syn region elixirMap matchgroup=elixirMapDelimiter start="%{" end="}" contains=ALLBUT,@elixirNotTop fold