Update agents.rst (#1324)

Correcting the link to the image of comparing algorithms (alg_compare.png)

Author: vnikoofard

Diff for: docs/source/start/three_layer/agents.rst

@@ -7,7 +7,7 @@ FinRL contains fine-tuned standard DRL algorithms in ElegantRL, Stable Baseline
 
 With those three powerful DRL libraries, FinRL provides the following algorithms for users:
 
-.. image:: ../image/alg_compare.png
+.. image:: ../../image/alg_compare.png
 
 As mentioned in the introduction, FinRL's DRL agents are built by fine-tuned standard DRL algorithms depending on three famous DRL library: ElegantRL, Stable Baseline 3, and RLlib.