Fix multiple file args

With multiple files, you would get a line for each file argument, but
only from the first file, never from any other file argument.

src/main.cpp

@@ -70,7 +70,7 @@ int main(int argc, char* argv[]) {
         for (int i = optind; i < argc; ++i) {
             try {
                 // process file argv[i]
-                ifstream infile {argv[1]};
+                ifstream infile {argv[i]};
                 cout << random_line_from_file(infile) << '\n';
             } catch (const runtime_error& ex) {
                 cerr << "ranlin: " << ex.what() << '\n';