fixing structure childmap

Co-authored-by: Pano Maria <[email protected]>

Author: dachafra

spec/section/joinconditions.md

@@ -55,7 +55,7 @@ The following RDF triples are generated by the [=RML mapping=] above.
 
 A <dfn data-lt="join">join condition</dfn> is represented by a resource that has exactly one value for each of the following two properties:
 
-* a child map (`rml:childMap`) property, whose value is a [=child map=].<br> A <dfn>child map</dfn> (`rml:ChildMap`) is an [=expression map=] which MUST be evaluated against the [=child logical source=]. The <dfn>child logical source</dfn> is the [=logical source=] of the [=triples map=] that contains the [=referencing object map=], i.e. the current [=triples map=], or it should have a [=constant value=].
+* a child map (`rml:childMap`) property, whose value is a [=child map=], or a [=constant-valued expression map=].<br> A <dfn>child map</dfn> (`rml:ChildMap`) is an [=expression map=] which MUST be evaluated against the [=child logical source=]. The <dfn>child logical source</dfn> is the [=logical source=] of the [=triples map=] that contains the [=referencing object map=], i.e. the current [=triples map=]..
 
 * a parent map (`rml:parentMap`) property, whose value is a [=parent map=].<br> A <dfn>parent map</dfn> (`rml:ParentMap`) is an [=expression map=], which MUST be evaluated against the [=parent logical source=]. The <dfn>parent logical source</dfn> is [=logical source=] of the [=referencing object map=]'s [=parent triples map=], i.e. the referenced [=triples map=], or it should have a [=constant value=].