[expr.mptr.oper] Add missing indentation in example code. [rejected upstream: cplusplus#1529]

Author: Eelis

source/expressions.tex

@@ -3973,9 +3973,9 @@
 };
 void f()
 {
-const S cs;
-int S::* pm = &S::i;            // \tcode{pm} refers to \tcode{mutable} member \tcode{S::i}
-cs.*pm = 88;                    // ill-formed: \tcode{cs} is a \tcode{const} object
+  const S cs;
+  int S::* pm = &S::i;          // \tcode{pm} refers to \tcode{mutable} member \tcode{S::i}
+  cs.*pm = 88;                  // ill-formed: \tcode{cs} is a \tcode{const} object
 }
 \end{codeblock}
 \end{note}