make code command line copyable

Update README.md
the command `dotnet fsi build.fsx -- --help` was indented 4 spaces to make it code block, but on github it apparently needs a blank line above it to box it out and make it have the "clipboard copy" button appear. 
The rendared page shows "Run dotnet fsi build.fsx -- --help" instead of a code block. 
Just added the additional blank line.

Author: Fxplorer

README.md

@@ -21,6 +21,7 @@ Once built, you can run the command-line tool to self-build the docs for this di
 ### Pipelines
 
 Run
+
     dotnet fsi build.fsx -- --help
 
 to see what other pipelines can be run from `build.fsx`.