docs: Use golib instead of goapp as the example (#117)

golib is a lot simpler example, goapp requires more variables.

Author: bendiknesbo

doc.go

@@ -21,11 +21,11 @@ module and mage as as tool.
 	go get github.com/coopnorge/mage@latest
 	go get -tool github.com/magefile/mage
 
-Configure mage in magefiles/magefile.go (goapp is used for the example).
+Configure mage in magefiles/magefile.go (golib is used for the example).
 
 	import (
 		//mage:import
-		_ "github.com/coopnorge/mage/targets/goapp"
+		_ "github.com/coopnorge/mage/targets/golib"
 	)
 
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