Update README.md for atest run --report args error (#45)

Author: haiker2011

README.md

@@ -47,7 +47,7 @@ Use "atest [command] --help" for more information about a command.
 
 below is an example of the usage, and you could see the report as well:
 
-`atest run -p sample/testsuite-gitlab.yaml --duration 1m --thread 3  --report m`
+`atest run -p sample/testsuite-gitlab.yaml --duration 1m --thread 3  --report md`
 
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