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

[ferrmin]

For golang#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

This PR will be imported into Gerrit with the title and first
comment (this text) used to generate the subject and body of
the Gerrit change.

Please ensure you adhere to every item in this list.

More info can be found at https://github.com/golang/go/wiki/CommitMessage

• The PR title is formatted as follows: net/http: frob the quux before blarfing
  • The package name goes before the colon
  • The part after the colon uses the verb tense + phrase that completes the blank in,
    "This change modifies Go to ___________"
  • Lowercase verb after the colon
  • No trailing period
  • Keep the title as short as possible. ideally under 76 characters or shorter
• No Markdown
• The first PR comment (this one) is wrapped at 76 characters, unless it's
  really needed (ASCII art, table, or long link)
• If there is a corresponding issue, add either Fixes #1234 or Updates #1234
  (the latter if this is not a complete fix) to this comment
• If referring to a repo other than golang/go you can use the
  owner/repo#issue_number syntax: Fixes golang/tools#1234
• We do not use Signed-off-by lines in Go. Please don't add them.
  Our Gerrit server & GitHub bots enforce CLA compliance instead.
• Delete these instructions once you have read and applied them