Replace √ (U+221A) with "OK"

The style of character U+221A is different among different platforms and
fonts.

doc/tutorials/connect/connect.md

@@ -7,7 +7,7 @@ This program will automatically display the currently available serial ports at
 ![1](1_zh_CN.png)  
 Double click the target device item on the left or manually enter/select the port name on the right, then enter or select the baud rate and click the "Open" button to connect.  
 ![2](2_zh_CN.png)  
-When the "Status" button below changes to √, the device is connected successfully.
+When the "Status" button below changes to OK, the device is connected successfully.
 ![3](3_zh_CN.png)  
 Before connecting to a device, you can click the "Advanced" checkbox to configure more parameters.  
 ![4](4_zh_CN.png)  
@@ -20,5 +20,5 @@ Each time a device is successfully opened, the program will save the parameters
 You can only connect to "Bonded" devices. For new devices, you need to pair them in the system Bluetooth settings first, then click the device you want to connect to and click the "Open" button on the right to connect  
 The "Bonded" devices in the list include devices that have been paired before but are not in the connection range, which might not be connectable.  
 ![7](7_zh_CN.jpg)  
-Click the "Open" button. When the "Status" button below changes to √, the device is connected successfully.  
+Click the "Open" button. When the "Status" button below changes to OK, the device is connected successfully.  
 The Android version can also quickly disconnect/reconnect the last connected device by clicking the "Status" button in the status bar.  

doc/tutorials/connect/connect_zh_CN.md

@@ -7,7 +7,7 @@
 ![1](1_zh_CN.png)  
 双击想要连接的设备或手动输入/选择端口名，再输入或选择波特率，单击右边的“打开”按钮即可连接  
 ![2](2_zh_CN.png)  
-下方“状态”变为√表明连接成功  
+下方“状态”变为OK表明连接成功  
 ![3](3_zh_CN.png)  
 连接设备之前可以单击“高级”复选框，进一步配置串口参数  
 ![4](4_zh_CN.png)  
@@ -20,5 +20,5 @@
 只能连接类型为“已配对”的设备。对于新设备，需要先在系统蓝牙设置当中配对，之后再单击想要连接的设备，单击右边的“打开”按钮进行连接  
 列表中的“已配对”设备包含之前配对过但是不在连接范围内的设备，并不一定可连接  
 ![7](7_zh_CN.jpg)  
-单击“打开”按钮后若下方“状态”变为√表明连接成功  
+单击“打开”按钮后若下方“状态”变为OK表明连接成功  
 Android端同样可以通过单击状态栏的“状态”按钮来快速断开/重连上次连接的设备  

src/mainwindow.cpp

@@ -334,7 +334,7 @@ void MainWindow::updateStatusBar()
     connArgsLabel->setText(connArgsText);
     Connection::State currState = IOConnection->state();
     if(currState == Connection::Connected)
-        stateButton->setText(tr("State") + ": √");
+        stateButton->setText(tr("State") + ": OK");
     else if(currState == Connection::Bound || currState == Connection::Connecting)
         stateButton->setText(tr("State") + ": ...");
     else