doc: remove file name from @file directive in Doxygen usage example

The file name is redundant and brittle across file renamings.

doc/developer.texi

@@ -92,7 +92,7 @@ above them explaining what the function does, even if it is just one sentence.
 All structures and their member variables should be documented, too.
 @example
 /**
- * @@file mpeg.c
+ * @@file
  * MPEG codec.
  * @@author ...
  */