[lex] Change index redirects from 'backslash' to 'backslash character'. (#1495)

Author: Eelis

source/lex.tex

@@ -1200,8 +1200,8 @@
 is supported for compatibility with ISO \CppXIV and ISO C.}
 and the backslash
 \indextext{backslash character}%
-\indextext{\idxcode{\textbackslash}|see{backslash}}%
-\indextext{escape character|see{backslash}}%
+\indextext{\idxcode{\textbackslash}|see{backslash character}}%
+\indextext{escape character|see{backslash character}}%
 \tcode{\textbackslash}, can be represented according to
 Table~\ref{tab:escape.sequences}.
 \indextext{escape sequence!undefined}%