[lex.string] Fix missing space in grammar production (#7577)

Author: OndrejPopp

source/lex.tex

@@ -1776,7 +1776,7 @@
 
 \begin{bnf}
 \nontermdef{d-char-sequence}\br
-    d-char\opt{d-char-sequence}
+    d-char \opt{d-char-sequence}
 \end{bnf}
 
 \begin{bnf}