MINOR Fix PR Linter "Reviewers" handling (apache#19415)

This patch fixes the textwrap for Reviewers to support the RFC 822
folding rules. From the RFC, headers or trailers can break across
multiple lines if there is a leading whitespace on subsequent lines.
This allows "git interpret-trailers" to correctly parse long
"Reviewers:" lines.

Reviewers: Lianet Magrans <[email protected]>

Author: mumrah

.github/scripts/pr-format.py

@@ -162,7 +162,10 @@ def check(positive_assertion, ok_msg, err_msg):
                 new_lines.append(textwrap.fill(line, width=72, break_long_words=False, break_on_hyphens=False, replace_whitespace=False))
             rewrapped_p = "\n".join(new_lines)
         else:
-            rewrapped_p = textwrap.fill("".join(p), width=72, break_long_words=False, break_on_hyphens=False, replace_whitespace=True)
+            indent = ""
+            if len(p) > 0 and p[0].startswith("Reviewers:"):
+                indent = " "
+            rewrapped_p = textwrap.fill("".join(p), subsequent_indent=indent, width=72, break_long_words=False, break_on_hyphens=False, replace_whitespace=True)
         new_paragraphs.append(rewrapped_p + "\n")
     body = "\n".join(new_paragraphs)