Add missing comma to string doc comment

Author: brianrourkeboll

src/FSharp.Core/prim-types.fsi

@@ -4784,7 +4784,7 @@ namespace Microsoft.FSharp.Core
 
         /// <summary>Converts the argument to a string using <c>ToString</c>.</summary>
         ///
-        /// <remarks>For standard integer and floating point values and any type that implements <c>IFormattable</c>
+        /// <remarks>For standard integer and floating point values and any type that implements <c>IFormattable</c>,
         /// <c>ToString</c> conversion uses <c>CultureInfo.InvariantCulture</c>. </remarks>
         /// <param name="value">The input value.</param>
         ///