AWS IAM tools: Fix comment

Fix a comment that explains how to set some environment variables that the
tool needs in order to start up. The path was incorrect and would cause
the tool to error out with a ClassNotFoundException.

Closes Homebrew/homebrew#24692.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/aws-iam-tools.rb

@@ -34,7 +34,7 @@ class AwsIamTools < AmazonWebServicesFormula
       * On Zsh, add them to `~/.zprofile` instead.
 
     export JAVA_HOME="$(/usr/libexec/java_home)"
-    export AWS_IAM_HOME="#{opt_prefix}/jars"
+    export AWS_IAM_HOME="#{opt_prefix}/libexec"
     export AWS_CREDENTIAL_FILE=$HOME/.aws-credentials-master
     EOS
   end