Add a link to Backslash escapes sub-section around reference to "backslash escapes" in zero-width space sub-section

Fixes: commonmark-spec#643

Author: haqer1

spec.txt

@@ -622,8 +622,8 @@ foo
 
 ## Zero-width space
 
-For most use-cases requiring escapes it's worth considering using backslash
-escapes, but zero-width space can be useful sometimes as well (to change the
+For most use-cases requiring escapes it's worth considering using [backslash
+escapes], but zero-width space can be useful sometimes as well (to change the
 meaning of an adjacent character).
 
 Given differences in utilization of whitespace, zero-width space can be used to