flag: changed flag variable name in package doc, for clarity

Changed the flag variable name to nFlag instead of flagname,
because flagname was confusing.

Change-Id: I20dd4c4b4f605395d427a125ba4fd14580e5d766
Reviewed-on: https://go-review.googlesource.com/c/go/+/221678
Reviewed-by: Rob Pike <r@golang.org>

flag/flag.go

@@ -9,9 +9,9 @@
 
 	Define flags using flag.String(), Bool(), Int(), etc.
 
-	This declares an integer flag, -flagname, stored in the pointer ip, with type *int.
+	This declares an integer flag, -n, stored in the pointer nFlag, with type *int:
 		import "flag"
-		var ip = flag.Int("flagname", 1234, "help message for flagname")
+		var nFlag = flag.Int("n", 1234, "help message for flag n")
 	If you like, you can bind the flag to a variable using the Var() functions.
 		var flagvar int
 		func init() {