gh-38616: `structure/sage_object.pyx`: remove excess `%s`s
To avoid the following warning:
```
warning: sage/structure/sage_object.pyx:722:42: Too few arguments for
format placeholders
```
URL: https://github.com/sagemath/sage/pull/38616
Reported by: gmou3
Reviewer(s): Matthias Köppe
