fix command slash

Author: riccid2

doc/nbgv-cli.md

@@ -25,7 +25,7 @@ If scripting for running in a CI build where global impact from installing a too
 dotnet tool install --tool-path . nbgv
 ```
 
-At this point you can launch the tool using `.\nbgv` in your build script.
+At this point you can launch the tool using `./nbgv` in your build script.
 
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