hclsyntax: Fix error diagnostic for blocks inside JustAttributes

The range was incorrectly being reported as "Context", rather than
"Subject". The Context field has meaning only in conjunction with Subject.

While here, this also tweaks the summary to show the block type name in
quotes, since otherwise the sentence can read oddly for certain block type
names.

hcl/hclsyntax/structure.go

@@ -255,9 +255,9 @@ func (b *Body) JustAttributes() (hcl.Attributes, hcl.Diagnostics) {
 		example := b.Blocks[0]
 		diags = append(diags, &hcl.Diagnostic{
 			Severity: hcl.DiagError,
-			Summary:  fmt.Sprintf("Unexpected %s block", example.Type),
+			Summary:  fmt.Sprintf("Unexpected %q block", example.Type),
 			Detail:   "Blocks are not allowed here.",
-			Context:  &example.TypeRange,
+			Subject:  &example.TypeRange,
 		})
 		// we will continue processing anyway, and return the attributes
 		// we are able to find so that certain analyses can still be done