doc(doc/build.md): fix the display problem of doc.

Author: SanftMonster

doc/Build.md

@@ -3,18 +3,20 @@
 1. Install ```mingw-w64``` and set the enviroment variable.
 2. clone the project.
 3. use the following command
-```
+
+```bash
 $ mkdir build
 $ cd build
 $ cmake .. -G "MinGW Makefiles"
 $ mingw32-make
-    ```
+```
 
 ## Cmake build on Linux
 
 1. clone the project.
 2. use the following command
-```
+
+```bash
 $ mkdir build
 $ cd build
 $ cmake ..
@@ -26,7 +28,8 @@ $ make
 1. Install ```mingw-w64``` and set the enviroment variable.
 2. clone the project.
 3. use the following command
-```
+
+```bash
 $ mkdir build
 $ cd build
 $ cmake .. -DNN_CROSS_COMPILE=ON