Info about image renderer plugin does not mention PMM admin

User impact:
Image renderer plugin info mentions Grafana admin instead of PMM admin

Steps to reproduce:

1. Start PMM Server
2. Open any dashboard
3. Open any panel menu
4. Select Share

Actual result:

• Image renderer plugin info says: 
• To render a panel image, you must install the Grafana image renderer plugin. Please contact your Grafana administrator to install the plugin.

Expected result:

• Info message should say:
• To render a panel image, you must install the Image Renderer plugin. Please contact your PMM administrator to install the plugin.