Fix struct comment typo (#28)

diff --git a/subcommands.go b/subcommands.go
index af8463a..39268f7 100644
--- a/subcommands.go
+++ b/subcommands.go
@@ -63,7 +63,7 @@
 	Error  io.Writer // Error specifies where the commander should write its error (default: os.Stderr).
 }
 
-// A commandGroup represents a set of commands about a common topic.
+// A CommandGroup represents a set of commands about a common topic.
 type CommandGroup struct {
 	name     string
 	commands []Command