README: fix link to x/tools

Change-Id: I4803bfe7da3b21fdfe503b9804015f0a5104a52e
Reviewed-on: https://go-review.googlesource.com/c/mod/+/601441
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

Author: mikesep

README.md

@@ -9,7 +9,7 @@ That is, it is for direct manipulation of Go modules themselves.
 It is NOT about supporting general development tools that
 need to do things like load packages in module mode.
 That use case, where modules are incidental rather than the focus,
-should remain in [x/tools](https://pkg.go.dev/golang/org/x/tools),
+should remain in [x/tools](https://pkg.go.dev/golang.org/x/tools),
 specifically [x/tools/go/packages](https://pkg.go.dev/golang.org/x/tools/go/packages).
 
 The specific case of loading packages should still be done by