[depr.lit] Fix grammar according to P2361R6

P2361R6 introduced the notion of unevaluated strings, and
updated the grammar for literal operator function accodingly.
Unfortunely, the corresponding grammar reference that was
deprecated was not similarly updated.

Author: AlisdairM

source/future.tex

@@ -171,7 +171,7 @@
 \pnum
 A \grammarterm{literal-operator-id}\iref{over.literal} of the form
 \begin{codeblock}
-operator @\grammarterm{string-literal}@ @\grammarterm{identifier}@
+operator @\grammarterm{unevaluated-string}@ @\grammarterm{identifier}@
 \end{codeblock}
 is deprecated.