doc/ref/spec.md: remove faulty use of \x in double-quoted string

Fixes #793

Change-Id: Ia95a0c6d3b9012ef441bda28307e1ad2ba6e74eb
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8906
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

doc/ref/spec.md

@@ -469,7 +469,7 @@ the string value.
 "Hello, \( name )!"
 "日本語"
 "\u65e5本\U00008a9e"
-"\xff\u00FF"
+'\xff\u00FF'
 "\uD800"             // illegal: surrogate half (TODO: probably should allow)
 "\U00110000"         // illegal: invalid Unicode code point
 
@@ -486,7 +486,7 @@ These examples all represent the same string:
 `日本語`                                 // UTF-8 input text as a raw literal
 "\u65e5\u672c\u8a9e"                    // the explicit Unicode code points
 "\U000065e5\U0000672c\U00008a9e"        // the explicit Unicode code points
-"\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e"  // the explicit UTF-8 bytes
+'\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e'  // the explicit UTF-8 bytes
 ```
 
 If the source code represents a character as two code points, such as a