Fix \xhh specs, #1889.

Doc/reference/lexical_analysis.rst

@@ -530,7 +530,7 @@ Notes:
    As in Standard C, up to three octal digits are accepted.
 
 (4)
-   Unlike in Standard C, at most two hex digits are accepted.
+   Unlike in Standard C, exactly two hex digits are required.
 
 (5)
    In a string literal, hexadecimal and octal escapes denote the byte with the