Build/Test Tools: Typo correction in rss2 unit tests.

Follow-up to [53233].

Props SergeyBiryukov.
Fixes #47968.


git-svn-id: https://develop.svn.wordpress.org/trunk@53241 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/feed/rss2.php

@@ -607,7 +607,7 @@ class Tests_Feed_RSS2 extends WP_UnitTestCase {
 				$this->assertSame(
 					strtotime( $headers['Last-Modified'] ),
 					strtotime( $today ),
-					'Last-Modified was not the date of the most recent most'
+					'Last-Modified was not the date of the most recent post'
 				);
 				return $headers;
 			}