[lex.string] Remove mention of trigraphs from raw string literal example.

Fixes #1635.

Author: zygoloid

source/lex.tex

@@ -1518,18 +1518,11 @@
 is equivalent to \tcode{"\textbackslash n)\textbackslash \textbackslash \textbackslash na\textbackslash"\textbackslash n"}. The raw string
 
 \begin{codeblock}
-R"(??)"
+R"(x = "\"y\"")"
 \end{codeblock}
 
-is equivalent to \tcode{"\textbackslash?\textbackslash?"}. The raw string
-
-\begin{codeblock}
-R"#(
-)??="
-)#"
-\end{codeblock}
-
-is equivalent to \tcode{"\textbackslash n)\textbackslash?\textbackslash?=\textbackslash"\textbackslash n"}. \end{example}
+is equivalent to \tcode{"x = \textbackslash "\textbackslash\textbackslash\textbackslash "y\textbackslash\textbackslash\textbackslash "\textbackslash ""}.
+\end{example}
 
 \pnum
 \indextext{string!type of}%