Add plus–minus sign

Author: xfq

maths/en.csv

@@ -68,6 +68,7 @@ origin,原点
 parallelogram,平行四边形
 piecewise-defined function,分段函数
 plane,平面
+plus–minus sign,正负号
 point,点
 polynomial,多项式
 prime,质数、素数

maths/en.html

@@ -375,6 +375,10 @@
             <td lang="en">plane</td>
             <td lang="zh-Hans">平面</td>
         </tr>
+        <tr>
+            <td lang="en">plus–minus sign</td>
+            <td lang="zh-Hans">正负号</td>
+        </tr>
         <tr>
             <td lang="en">point</td>
             <td lang="zh-Hans">点</td>