Update README.md

Author: tigerblue77

README.md

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+> **Warning** If you update to the latest version, be sure to replace "CPU_TEMPERATURE_TRESHOLD" environment variable with "CPU_TEMPERATURE_T<ins>H</ins>RESHOLD" which was a typo
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 # Dell iDRAC fan controller Docker image
 Download Docker image from :
 - [Docker Hub](https://hub.docker.com/r/tigerblue77/dell_idrac_fan_controller)