Fix small typo: HRM --> HMR (#995)

Author: gasi

README.md

@@ -670,7 +670,7 @@ TheRubyRacer [hasn't updated LibV8](https://github.com/cowboyd/therubyracer/blob
 LibV8 itself is already [beyond version 7](https://github.com/cowboyd/libv8/releases/tag/v7.3.492.27.1) therefore many serverside issues are caused by old JS engines and fixed by using an up to date one such as [MiniRacer](https://github.com/discourse/mini_racer) or [TheRubyRhino](https://github.com/cowboyd/therubyrhino) on JRuby.
 
 ### HMR
-HRM is possible with this gem as it does just pass through to Webpacker. Please open an issue to let us know tips and tricks for it to add to the wiki.
+HMR is possible with this gem as it does just pass through to Webpacker. Please open an issue to let us know tips and tricks for it to add to the wiki.
 
 One example: [Stack Overflow answer with Babel and Webpacker config](https://stackoverflow.com/a/54846330/193785)