Fixed the header spacer element in header-center template (#5175)

blockbase/block-template-parts/header-centered.html

@@ -16,7 +16,7 @@
 <!-- /wp:spacer -->
 <!-- wp:navigation {"__unstableLocation":"primary","layout":{"type":"flex","setCascadingProperties":true,"justifyContent":"center","orientation":"horizontal"}} /-->
 
-<!-- wp:template-part {"slug":"header-spacer","theme":"themes/seedlet-blocks"} /-->
+<!-- wp:template-part {"slug":"header-spacer"} /-->
 
 </div>
 <!-- /wp:group -->