Typo correction in flag.go (#164)

flag.go

diff --git a/flag.go b/flag.go
index e7f6779..5eadc84 100644
--- a/flag.go
+++ b/flag.go
@@ -279,7 +279,7 @@
 	}
 }
 
-// HasFlags returns a bool to indicate if the FlagSet has any flags definied.
+// HasFlags returns a bool to indicate if the FlagSet has any flags defined.
 func (f *FlagSet) HasFlags() bool {
 	return len(f.formal) > 0
 }