docs: close Spanner instance when it is no longer needed (#1116)

The CreateInstance example opened a Spanner instance without closing it.
That was spamming the test logs with a lot of warnings.

samples/snippets/src/main/java/com/example/spanner/CreateInstanceExample.java

@@ -65,6 +65,8 @@ static void createInstance(String projectId, String instanceId) {
           instanceInfo.getId(), e.getMessage());
     } catch (InterruptedException e) {
       System.out.println("Error: Waiting for createInstance operation to finish was interrupted");
+    } finally {
+      spanner.close();
     }
   }
 }