Fix incorrect example in spl_cache

= Testing

```
sdb> man spl_cache
SUMMARY
    spl_cache [-h]

...<cropped>...

EXAMPLES
    Print all the objects in the ddt_cache:

        sdb> spl_kmem_caches | filter 'obj.skc_name == "ddt_cache"' | spl_cache
        (void *)0xffffa08937e80040
        (void *)0xffffa08937e86180
        (void *)0xffffa08937e8c2c0
        (void *)0xffffa08937e92400
        ...
```

= Github Issue Tracker Automation

Closes #295

sdb/commands/spl/spl_kmem_caches.py

@@ -212,7 +212,7 @@ class SplKmemCacheWalker(sdb.Walker):
     EXAMPLES
         Print all the objects in the ddt_cache:
 
-            sdb> spl_kmem_caches | filter obj.skc_name == "ddt_cache" | spl_cache
+            sdb> spl_kmem_caches | filter 'obj.skc_name == "ddt_cache"' | spl_cache
             (void *)0xffffa08937e80040
             (void *)0xffffa08937e86180
             (void *)0xffffa08937e8c2c0