go/analysis/passes/fieldalignment: fix doc

"go fieldalignment" is not a command that works.

Change-Id: I0eda2c0491c26c30c6661be96d3f6c7e5db920ff
Reviewed-on: https://go-review.googlesource.com/c/tools/+/607295
Reviewed-by: Tim King <taking@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Tim King <taking@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

go/analysis/passes/fieldalignment/fieldalignment.go

@@ -53,7 +53,7 @@ so the analyzer is not included in typical suites such as vet or
 gopls. Use this standalone command to run it on your code:
 
    $ go install golang.org/x/tools/go/analysis/passes/fieldalignment/cmd/fieldalignment@latest
-   $ go fieldalignment [packages]
+   $ fieldalignment [packages]
 
 `