[re.grammar] Add missing backslash to UnicodeEscapeSequence example (cplusplus#7290)

Author: jwakely

source/regex.tex

@@ -3929,7 +3929,7 @@
 value that can be held in an object of type \tcode{charT} the translator shall
 throw an exception object of type \tcode{regex_error}.
 \begin{note}
-This means that values of the form \tcode{"uxxxx"} that do not fit in
+This means that values of the form \tcode{"\textbackslash{}uxxxx"} that do not fit in
 a character are invalid.
 \end{note}