Fixed a syntax issue

Author: javiereguiluz

service_container/tags.rst

@@ -64,7 +64,7 @@ tag: the container sees that your class extends ``AbstractExtension`` (or more a
 that it implements ``ExtensionInterface``) and adds the tag for you.
 
 If you want to apply tags automatically for your own services, use the
-``_instanceof`` option::
+``_instanceof`` option:
 
 .. configuration-block::