[DOCS] #4587 – Make Sphinx warnings break docs build

Add blank line to prevent Sphinx warning "WARNING: Explicit markup ends without a blank line; unexpected unindent.".

docs/internals/frontends.rst

@@ -272,6 +272,7 @@ stats
         Compressed and encrypted size of all chunks
 
 .. highlight: json
+
 Example *borg info* output::
 
     {