Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -38,5 +38,5 @@ where:
 &nbsp;&nbsp; :small_orange_diamond: <a href="https://drive.google.com/file/d/1zdIDyV8qDBWNmmlwhBw7EtLu_pyacdOh/view"><code>book</code></a> <a href="https://github.com/8128/SharedResources/blob/master/Introduction%20to%20Linear%20Algebra%205th%202016/Introduction%20to%20Linear%20Algebra%2C%205th%20%20(Solutions)%20%E2%80%93%202016.pdf"><code>solution</code></a> <a href="https://math.mit.edu/~gs/linearalgebra/ila5/indexila5.html"><code>material</code></a> <a href="https://www.mathworks.com/matlabcentral/fileexchange/2166-introduction-to-linear-algebra"><code>code</code></a> <a href="https://github.com/shahrokh-bahtooei/Linear-Algebra-Gilbert-Strang"><code>code</code></a> <b>Introduction to Linear Algebra</b> - Gilbert Strang. 5th edition.<br><br>
 In this example, we have a link to the book, one solution manual, one material, and two code solutions which are all related to the book "Introduction to Linear Algebra". We gain conciseness at the cost of lack of details. When using multiple tag, follow these guidelines:
     - **You should use multiple tags in a same item only if their contents are closely related and if the relationship is easy to infer** (in the previous example, it is easy to understand that all links refer to the same book). If one of these conditions is not met, split the suggestion in multiple items using only one tag per item.
-    - Don't try explain all tags in the short title or in the longer explanation. Instead, stick with the main tag, which is the first tag of the item (in the previous example, the main tag is `book`). All other tags add must be related to the main tag.
+    - Don't try to explain all tags in the short title or in the longer explanation. Instead, stick with the main tag, which is the first tag of the item (in the previous example, the main tag is `book`). All other tags add must be related to the main tag.
     - Although we can repeat a same tag multiple times if necessary, you cannot repeat the main tag multiple times. In the previous example, you can use as many `software-tool`, `solution`, etc... as you need, but you cannot add another `book` tag.