Explicitly allow literal-to-literal mappings.

Change the phrasing of the description of "literal mappings" to make it
clear that it is perfectly feasible and allowed to have literal on
_both_ sides of a mapping.

src/docs/spec-model.md

@@ -96,7 +96,7 @@ In addition, predicates from the following sources MAY also be encouraged:
 
 <a id="literal-mappings"></a>
 
-The SSSOM model is primarily intended to represent mappings between semantic entities. However, it may also be used to represent mappings where one side is a literal string that does not have an identifier of its own. Any such mapping is henceforth called a _literal mapping_.
+The SSSOM model is primarily intended to represent mappings between semantic entities. However, it may also be used to represent mappings where at least one side is a literal string that does not have an identifier of its own. Any such mapping is henceforth called a _literal mapping_.
 
 To represent a mapping whose subject (resp. object) is a literal: