doc/ref/spec.md: fix escaping

Fixes #795

Change-Id: Idb9ac23996f5cfc49d7aca845ec1c0759f7f7e20
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8905
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

doc/ref/spec.md

@@ -431,13 +431,13 @@ All other sequences starting with a backslash are illegal inside literals.
 ```
 escaped_char     = `\` { `#` } ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | "/" | `\` | "'" | `"` ) .
 byte_value       = octal_byte_value | hex_byte_value .
-octal_byte_value = `\` octal_digit octal_digit octal_digit .
-hex_byte_value   = `\` "x" hex_digit hex_digit .
-little_u_value   = `\` "u" hex_digit hex_digit hex_digit hex_digit .
-big_u_value      = `\` "U" hex_digit hex_digit hex_digit hex_digit
+octal_byte_value = `\` { `#` } octal_digit octal_digit octal_digit .
+hex_byte_value   = `\` { `#` } "x" hex_digit hex_digit .
+little_u_value   = `\` { `#` } "u" hex_digit hex_digit hex_digit hex_digit .
+big_u_value      = `\` { `#` } "U" hex_digit hex_digit hex_digit hex_digit
                            hex_digit hex_digit hex_digit hex_digit .
 unicode_value    = unicode_char | little_u_value | big_u_value | escaped_char .
-interpolation    = "\(" Expression ")" .
+interpolation    = "\" { `#` } "(" Expression ")" .
 
 string_lit       = simple_string_lit |
                    multiline_string_lit |