Replace InvalidPathChars for NS1.3

* InvalidPathChars does not exist in NS1.3; replaced with GetInvalidPathChars instead.

src/Discord.Net.Rest/Entities/Channels/RestDMChannel.cs

@@ -81,7 +81,7 @@ namespace Discord.Rest
         /// <inheritdoc />
         /// <exception cref="ArgumentException">
         /// <paramref name="filePath" /> is a zero-length string, contains only white space, or contains one or more
-        /// invalid characters as defined by <see cref="System.IO.Path.InvalidPathChars" />.
+        /// invalid characters as defined by <see cref="System.IO.Path.GetInvalidPathChars" />.
         /// </exception>
         /// <exception cref="ArgumentNullException">
         /// <paramref name="filePath" /> is <c>null</c>.

src/Discord.Net.Rest/Entities/Channels/RestGroupChannel.cs

@@ -98,7 +98,7 @@ namespace Discord.Rest
         /// <inheritdoc />
         /// <exception cref="ArgumentException">
         /// <paramref name="filePath" /> is a zero-length string, contains only white space, or contains one or more
-        /// invalid characters as defined by <see cref="System.IO.Path.InvalidPathChars" />.
+        /// invalid characters as defined by <see cref="System.IO.Path.GetInvalidPathChars" />.
         /// </exception>
         /// <exception cref="ArgumentNullException">
         /// <paramref name="filePath" /> is <c>null</c>.

src/Discord.Net.Rest/Entities/Channels/RestTextChannel.cs

@@ -79,7 +79,7 @@ namespace Discord.Rest
         /// <inheritdoc />
         /// <exception cref="ArgumentException">
         /// <paramref name="filePath" /> is a zero-length string, contains only white space, or contains one or more
-        /// invalid characters as defined by <see cref="System.IO.Path.InvalidPathChars" />.
+        /// invalid characters as defined by <see cref="System.IO.Path.GetInvalidPathChars" />.
         /// </exception>
         /// <exception cref="ArgumentNullException">
         /// <paramref name="filePath" /> is <c>null</c>.