[cpp.embed.gen] Add missing \tcode

Author: tkoeppe

source/preprocessor.tex

@@ -1025,7 +1025,7 @@
 name is replaced by its replacement list of preprocessing tokens).
 Then, an attempt is made to form a \grammarterm{header-name}
 preprocessing token\iref{lex.header} from the whitespace and the characters
-of the spellings of the resulting sequence of preprocessing tokens immediately after embed;
+of the spellings of the resulting sequence of preprocessing tokens immediately after \tcode{embed};
 the treatment of whitespace
 is \impldef{treatment of whitespace when processing a \tcode{\#embed} directive}.
 If the attempt succeeds, the directive with the so-formed \grammarterm{header-name}