[algorithms.parallel.defns] Fix indentation in example (#6707)

Author: brycelelbach

source/algorithms.tex

@@ -348,7 +348,7 @@
   int a[] = {1,2};
   std::for_each(std::execution::par_unseq, std::begin(a), std::end(a), [&](int) {
     std::lock_guard<mutex> guard(m);            // incorrect: \tcode{lock_guard} constructor calls \tcode{m.lock()}
-  ++x;
+    ++x;
   });
 }
 \end{codeblock}