Fix typo in description of raw strings: "value" --> "valid"
This closes SF bug #443059.
diff --git a/Doc/ref/ref2.tex b/Doc/ref/ref2.tex
index 8a44b03..e912242 100644
--- a/Doc/ref/ref2.tex
+++ b/Doc/ref/ref2.tex
@@ -403,7 +403,7 @@
`n'. String quotes can be escaped with a backslash, but the backslash
remains in the string; for example, \code{r"\e""} is a valid string
literal consisting of two characters: a backslash and a double quote;
-\code{r"\e"} is not a value string literal (even a raw string cannot
+\code{r"\e"} is not a valid string literal (even a raw string cannot
end in an odd number of backslashes). Specifically, \emph{a raw
string cannot end in a single backslash} (since the backslash would
escape the following quote character). Note also that a single