[sledge] Fix trace spec parsing of functions starting with underscore

Reviewed By: ngorogiannis

Differential Revision: D24989069

fbshipit-source-id: e90ae4f45

sledge/ppx_trace/trace/trace.ml

@@ -8,7 +8,7 @@
 module Char = struct
   include Char
 
-  let is_lowercase = function 'a' .. 'z' -> true | _ -> false
+  let is_lowercase = function 'a' .. 'z' | '_' -> true | _ -> false
   let is_uppercase = function 'A' .. 'Z' -> true | _ -> false
 end