cmd/go: remove a duplicated word from 'go help mod graph'

For #46366

Change-Id: Ie9735027a3c4c0f4a604df30ca4d64dcdc62b45a
Reviewed-on: https://go-review.googlesource.com/c/go/+/334375
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

Author: Bryan C. Mills

src/cmd/go/alldocs.go

@@ -26,7 +26,7 @@ in text form. Each line in the output has two space-separated fields: a module
 and one of its requirements. Each module is identified as a string of the form
 path@version, except for the main module, which has no @version suffix.
 
-The -go flag causes graph to report the module graph as loaded by by the
+The -go flag causes graph to report the module graph as loaded by the
 given Go version, instead of the version indicated by the 'go' directive
 in the go.mod file.