doc/ref/spec.md: bug fix: add solidus to escaped_char

Implementation already implements this. Spec still
has Go definition.

The solidus is a valid escape char in JSON.

Change-Id: Iabb7188fc0a29ba022f088c0f4b8f2145952b76f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3869
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/doc/ref/spec.md b/doc/ref/spec.md
index 4666355..22fc29c 100644
--- a/doc/ref/spec.md
+++ b/doc/ref/spec.md
@@ -430,7 +430,7 @@
 All other sequences starting with a backslash are illegal inside literals.
 
 ```
-escaped_char     = `\` { `#` } ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .
+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 .