[lex.ext] Simplify use of \placeholder. (cplusplus#4091)

Author: Eelis

source/lex.tex

@@ -1915,7 +1915,7 @@
 If \placeholder{S} contains a raw literal operator,
 the literal \placeholder{L} is treated as a call of the form
 \begin{codeblock}
-operator "" @\placeholder{X}@(@"\placeholder{n}{"}@)
+operator "" @\placeholder{X}@("@\placeholder{n}@")
 \end{codeblock}
 Otherwise (\placeholder{S} contains a numeric literal operator template),
 \placeholder{L} is treated as a call of the form
@@ -1941,7 +1941,7 @@
 If \placeholder{S} contains a raw literal operator,
 the \grammarterm{literal} \placeholder{L} is treated as a call of the form
 \begin{codeblock}
-operator "" @\placeholder{X}@(@"\placeholder{f}{"}@)
+operator "" @\placeholder{X}@("@\placeholder{f}@")
 \end{codeblock}
 Otherwise (\placeholder{S} contains a numeric literal operator template),
 \placeholder{L} is treated as a call of the form