docs, src: fixing man page and usage text readability

src/dj.c

@@ -314,8 +314,8 @@ static int
 usage(void){
 
 	fprintf(stderr, "Usage: %s [-AdfHqQ] [-a byte] [-c count]\n"
-		"\t[-i input_file] [-b input_block_size] [-s input_offset]\n"
-		"\t[-o output_file] [-B output_block_size] [-S output_offset]\n",
+		"\t[-i file] [-b block_size] [-s offset]\n"
+		"\t[-o file] [-B block_size] [-S offset]\n",
 		program_name);
 
 	return EX_USAGE;