Add "txt" highlighting to Markdown blocks.

This ensures it's styled with a differently-coloured background.

Author: bgrainger

docs/content/tutorials/connect-to-mysql.md

@@ -18,7 +18,7 @@ This tutorial will teach you how to connect to MySQL from .NET Core using C#.
 First, install the [MySqlConnector NuGet package](https://www.nuget.org/packages/MySqlConnector/). From
 a command prompt, run:
 
-```
+```txt
 dotnet add package MySqlConnector
 ```
 

docs/content/tutorials/dapper.md

@@ -32,7 +32,7 @@ You will need to install the following NuGet packages:
 
 Build your connection string by substituting the appropriate values in this template:
 
-```
+```txt
 Server=YOURSERVER; User ID=YOURUSERID; Password=YOURPASSWORD; Database=YOURDATABASE
 ```
 

docs/content/tutorials/efcore.md

@@ -22,7 +22,7 @@ It uses MySqlConnector to provide the core database connectivity.
 In this tutorial, we'll use a console application to demonstrate the core concepts.
 Create a console application, then add the Pomelo.EntityFrameworkCore.MySql NuGet package:
 
-```
+```txt
 dotnet new console -o EFCoreMySQL
 cd EFCoreMySQL
 dotnet add package Pomelo.EntityFrameworkCore.MySql
@@ -32,7 +32,7 @@ dotnet add package Pomelo.EntityFrameworkCore.MySql
 
 Build your connection string by substituting the appropriate values in this template:
 
-```
+```txt
 Server=YOURSERVER; User ID=YOURUSERID; Password=YOURPASSWORD; Database=YOURDATABASE
 ```