treewide: Remove indefinite article from meta.description

nix run nixpkgs#silver-searcher -- -G '\.nix$' -0l 'description.*"[Aa]n?' pkgs \
  | xargs -0 nix run nixpkgs#gnused -- -i '' -Ee 's/(description.*")[Aa]n? (.)/\1\U\2/'

pkgs/development/python2-modules/hypothesis/default.nix

@@ -38,7 +38,7 @@ buildPythonPackage rec {
   '';
 
   meta = with lib; {
-    description = "A Python library for property based testing";
+    description = "Python library for property based testing";
     homepage = "https://github.com/HypothesisWorks/hypothesis";
     license = licenses.mpl20;
   };