doc/ref: one char type fixup

Change-Id: Ica1786c0d3a805efea01602763b700fda2dcd2fa
diff --git a/doc/ref/spec.md b/doc/ref/spec.md
index afde726..2d30c51 100644
--- a/doc/ref/spec.md
+++ b/doc/ref/spec.md
@@ -1798,7 +1798,7 @@
 mpvl: they are not, x must be an instance of T, in which case x == T&x,
 so unification would be unnecessary.
 -->
-<!-->
+<!--
 ```
 T: {
     a: { b: 1..10 }