add oxford comma back in

Co-authored-by: Liam Cervante <liam.cervante@hashicorp.com>

Author: DanielMSchmidt

internal/lang/eval.go

@@ -312,7 +312,7 @@ func (s *Scope) evalContext(refs []*addrs.Reference, selfAddr addrs.Referenceabl
 					// this codepath doesn't really "know about". If the "self"
 					// object starts being supported in more contexts later then
 					// we'll need to adjust this message.
-					Detail:  `The "self" object is not available in this context. This object can be used only in resource provisioner, connection and postcondition blocks.`,
+					Detail:  `The "self" object is not available in this context. This object can be used only in resource provisioner, connection, and postcondition blocks.`,
 					Subject: ref.SourceRange.ToHCL().Ptr(),
 				})
 				continue

internal/terraform/evaluate_valid.go

@@ -64,7 +64,7 @@ func (e *Evaluator) staticValidateReference(ref *addrs.Reference, modCfg *config
 				// this codepath doesn't really "know about". If the "self"
 				// object starts being supported in more contexts later then
 				// we'll need to adjust this message.
-				Detail:  `The "self" object is not available in this context. This object can be used only in resource provisioner, connection and postcondition blocks.`,
+				Detail:  `The "self" object is not available in this context. This object can be used only in resource provisioner, connection, and postcondition blocks.`,
 				Subject: ref.SourceRange.ToHCL().Ptr(),
 			})
 			return diags