Update comment on ListPullRequestsWithCommit API

[mengmichael1]

1. Correct the statement that results include open and closed PRs by default, when it is only open by default, per

   go-github/github/pulls.go

   Lines 120 to 123 in ff33a55

   	typePullRequestListOptionsstruct {
   	// State filters pull requests based on their state. Possible values are:
   	// open, closed, all. Default is "open".
   	Statestring`url:"state,omitempty"`

2. Correct the URL for the Github API doc, per ListPullRequestsWithCommit removed from API docs?  #2227

[codecov]

Codecov Report

Merging #2228 (854edb5) into master (ff33a55) will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #2228 +/- ##
=======================================
Coverage 97.81% 97.81% =======================================
Files 112 112 Lines 10117 10117 =======================================
Hits 9896 9896 Misses 154 154 Partials 67 67 

Impacted Files	Coverage Δ
github/pulls.go	96.77% <ø> (ø)

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[mengmichael1]

PTAL @gmlewis, as you pointed me in the right direction for the API docs :) (also saw you helped out with #1186)

Also -- are you a repo manager or can you introduce me to one?

🔁 Repository managers: You can rescan this PR by pressing the "Re-run" button to the left of this description.

I signed the CLA after putting up this PR, so will need a repo manager to re-run the cla/google check for me

[gmlewis]

Yes, I am the primary maintainer of this repo.

[gmlewis]

Thank you, @mengmichael1 !
LGTM.
Merging.