Fixed issue serilog-mssql#541 in README.md

The Timestamp column is nullable by default but the README.md stated the opposite. This was fixed now.

README.md

@@ -188,7 +188,7 @@ CREATE TABLE [Logs] (
    [Message] nvarchar(max) NULL,
    [MessageTemplate] nvarchar(max) NULL,
    [Level] nvarchar(max) NULL,
-   [TimeStamp] datetime NOT NULL,
+   [TimeStamp] datetime NULL,
    [Exception] nvarchar(max) NULL,
    [Properties] nvarchar(max) NULL