cmd/link: improve -X doc wording

Change-Id: I9329738ec5dd6c12f72eea7de413d66617b7b5c2
Reviewed-on: https://go-review.googlesource.com/34373Reviewed-by: Rob Pike <r@golang.org>

src/cmd/link/doc.go

@@ -41,7 +41,7 @@ Flags:
 		Set the value of the string variable in importpath named name to value.
 		Note that before Go 1.5 this option took two separate arguments.
 		Now it takes one argument split on the first = sign.
-		To figure out the name to use, run `go tool nm PROGRAM`;
+		To discover which name to use, run `go tool nm PROGRAM`;
 		usable variables will be tagged "D" or "d".
 	-buildmode mode
 		Set build mode (default exe).