bump mdoc to 5.9.1

mdoc/Consts.cs

@@ -3,7 +3,7 @@ namespace Mono.Documentation
 {
     public static class Consts
     {
-        public static string MonoVersion = "5.9.0";
+        public static string MonoVersion = "5.9.1";
         public const string DocId = "DocId";
         public const string CppCli = "C++ CLI";
         public const string CppCx = "C++ CX";

mdoc/mdoc.nuspec

@@ -2,7 +2,7 @@
 <package >
   <metadata>
     <id>mdoc</id>
-    <version>5.9.0</version>
+    <version>5.9.1</version>
     <title>mdoc</title>
     <authors>Microsoft</authors>
     <owners>Microsoft</owners>