fix sample image width and height

Author: lomik

README.md

@@ -7,7 +7,7 @@ PNG renderer for [prometheus](https://github.com/prometheus/prometheus) based on
 ```
 http://localhost:8080/?query=min(noolite_rx_sensor_temp_celsius)by(ch)&from=-7d&width=600&height=400&areaMode=all&areaAlpha=0.09&title=Temp&bgcolor=161619&colorList=7EB26D,EAB839,6ED0E0,EF843C,E24D42,1F78C1,BA43A9,705DA0,508642,CCA300,447EBC,C15C17,890F02,0A437C,6D1F62,584477,B7DBAB,F4D598,70DBED,F9BA8F,F29191,82B5D8,E5A8E2,AEA2E0,629E51,E5AC0E,64B0C8,E0752D,BF1B00,0A50A1,962D82,614D93,9AC48A,F2C96D,65C5DB,F9934E,EA6460,5195CE,D683CE,806EB7,3F6833,967302,2F575E,99440A,58140C,052B51,511749,3F2B5B,E0F9D7,FCEACA,CFFAFF,F9E2D2,FCE2DE,BADFF4,F9D9F9,DEDAF7
 ```
-![graph.png](graph2.png)
+<img src="graph2.png" width="600" height="400">
 
 ## Run
 ```