fix indent in load-balancer.md (#980)

* fix indent

* 1. fix another indent
2. demonstrate trimPrefix as well as addPrefix in the cookbook.

---------

Co-authored-by: Xu Dongyan <dongyanxu@189.cn>

Author: qdongxu

doc/cookbook/load-balancer.md

@@ -59,7 +59,7 @@ filters:
     host: easegress.megaease.com
     method: POST
     path:
-    addPrefix: /apis/v2
+      addPrefix: /apis/v2
     header:
       set:
         X-Api-Version: v2
@@ -96,7 +96,7 @@ filters:
     host: easegress.megaease.com
     method: POST
     path:
-    addPrefix: /apis/v2
+      trimPrefix: /apis/v2
     header:
       set:
         X-Api-Version: v2