acme: reword the godoc to be more human readable

Updates #21902

Change-Id: I2988ba001450339d196da3a0f0687897e807e886
Reviewed-on: https://go-review.googlesource.com/64090
Reviewed-by: JBD <jbd@google.com>

acme/acme.go

@@ -257,7 +257,7 @@ func (c *Client) RevokeCert(ctx context.Context, key crypto.Signer, cert []byte,
 func AcceptTOS(tosURL string) bool { return true }
 
 // Register creates a new account registration by following the "new-reg" flow.
-// It returns registered account. The a argument is not modified.
+// It returns registered account. The account is not modified.
 //
 // The registration may require the caller to agree to the CA's Terms of Service (TOS).
 // If so, and the account has not indicated the acceptance of the terms (see Account for details),