Fix FloggerArgumentToString bug pattern flagged by error prone

Details: https://errorprone.info/bugpattern/FloggerArgumentToString
Change-Id: I20e756c4de965964c1d6a301a20913c6d2a4153e

src/main/java/com/googlesource/gerrit/plugins/webhooks/PostTask.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/webhooks/PostTask.java b/src/main/java/com/googlesource/gerrit/plugins/webhooks/PostTask.java
index 354bdf8..c069e3a 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/webhooks/PostTask.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/webhooks/PostTask.java
@@ -88,7 +88,7 @@
         logRetry(e);
         reschedule();
       } else {
-        log.atSevere().withCause(e).log("Failed to post: %s", toString());
+        log.atSevere().withCause(e).log("Failed to post: %s", this);
       }
     }
   }
@@ -98,12 +98,11 @@
   }
 
   private void logRetry(String reason) {
-    log.atFine().log(
-        "Retrying %s in %dms. Reason: %s", toString(), remote.getRetryInterval(), reason);
+    log.atFine().log("Retrying %s in %dms. Reason: %s", this, remote.getRetryInterval(), reason);
   }
 
   private void logRetry(Throwable cause) {
-    log.atFine().withCause(cause).log("Retrying %s in %dms", toString(), remote.getRetryInterval());
+    log.atFine().withCause(cause).log("Retrying %s in %dms", this, remote.getRetryInterval());
   }
 
   @Override