[range.split.overview] Improve string splitting example

With the adoption of P1989R2, `string_view` is constructible from
`subrange`. We can use that to simplify the example of string splitting.

From  pull request #4714.

Author: cor3ntin

source/ranges.tex

@@ -6192,10 +6192,8 @@
 \begin{example}
 \begin{codeblock}
 string str{"the quick brown fox"};
-for (span<char const> word : split(str, ' ')) {
-  for (char ch : word)
-    cout << ch;
-  cout << '*';
+for (string_view word : split(str, ' ')) {
+  cout << word << '*';
 }
 // The above prints: the*quick*brown*fox*
 \end{codeblock}