Corrected New Contributor Guide PR merging details (#10101)

### Summary
The New Contributor Guide added in [this
PR](#9977) had an inaccuracy,
mentioned in [this
comment](#9977 (comment))
from @byjlw. The guide mistakenly said that only core contributors can
merge PRs.

This PR corrects that error. It encourages the reader to merge their PR
themselves once approved, and provides illustrative screenshots.

Co-authored-by: jhels <[email protected]>

Author: jhelsby

docs/source/_static/img/new-contributor-guide/pr_approval1.png

@@ -271,6 +271,14 @@ Once you've successfully finished local development, it's time to send out your
 
     Those reviewers/maintainers are here to finetune your contribution and eventually catch some issues before we merge the PR. We aim for this process to be pleasing on both sides: we try to give and get the best.
 
-    Once the reviewers are happy, one of our core contributors will merge your PR. Congratulations — you're now an official ExecuTorch contributor!
+    Once the reviewers are happy, they'll approve your PR, indicating that they're happy for it to be merged. This will send you a notification and display as follows on your PR page:
+
+    ![](./_static/img/new-contributor-guide/pr_approval1.png)
+
+    And in the PR comments:
+
+    ![](./_static/img/new-contributor-guide/pr_approval2.png)
+
+    Once you've received the required approval from a core contributor, you can go ahead and merge your PR. Congratulations — you're now an official ExecuTorch contributor!
 
 Great job making it to the end of our guide — we hope you enjoy contributing. Once again, please check out **[Discord Server](https://discord.com/invite/Dh43CKSAdc)** if you want to say hello, ask any questions, or talk about any and all things ExecuTorch. We look forward to receiving your contributions!