[range.adjacent.overview] Use tuple in example, not pair (#5779)

adjacent_view always yields tuples, but the example was written as if it yielded a std::pair.

Author: cor3ntin

source/ranges.tex

@@ -10548,7 +10548,7 @@
 vector v = {1, 2, 3, 4};
 
 for (auto i : v | views::adjacent<2>) {
-  cout << "(" << i.first << ", " << i.second << ") ";   // prints \tcode{(1, 2) (2, 3) (3, 4)}
+  cout << "(" << std::get<0>(i) << ", " << std::get<1>(i) << ") ";  // prints \tcode{(1, 2) (2, 3) (3, 4)}
 }
 \end{codeblock}
 \end{example}