[5.0.x] Fixed grammatical error in stub release notes for upcoming security release.

Backport of b941de3 from main.

Author: nessita

docs/releases/4.2.16.txt

@@ -5,6 +5,6 @@ Django 4.2.16 release notes
 *September 3, 2024*
 
 Django 4.2.16 fixes one security issue with severity "moderate" and one
-security issues with severity "low" in 4.2.15.
+security issue with severity "low" in 4.2.15.
 
 ...

docs/releases/5.0.9.txt

@@ -5,6 +5,6 @@ Django 5.0.9 release notes
 *September 3, 2024*
 
 Django 5.0.9 fixes one security issue with severity "moderate" and one security
-issues with severity "low" in 5.0.8.
+issue with severity "low" in 5.0.8.
 
 ...