tutorial: Update CNI plugins install example

Commit containernetworking/plugins@4e1f780 (Split build.sh into two
OS-specific scripts) removes the build.sh file. Now we have
build_linux.sh and build_windows.sh

Signed-off-by: Radostin Stoyanov <rstoyanov1@gmail.com>

tutorial.md

@@ -223,7 +223,7 @@ cd $GOPATH/src/github.com/containernetworking/plugins
 Build the `CNI` plugins:
 
 ```
-./build.sh
+./build_linux.sh # or build_windows.sh
 ```
 
 Output: