Fix typo of "舌" (#551)

* Replace "舍背" with "舌背"

* Replace "舍尾" with "舌尾"

* Replace "舍末" with "舌末"

* Replace "舍" with "舌"

1000-hours/sounds-of-american-english/2.2.4-kg.md

@@ -4,7 +4,7 @@
 
 Thankfully, these two sounds exist in almost every language. Otherwise, learning them could prove quite challenging.
 
-首先，这两个音需要舍背后部抬起来顶住软腭而后开始发音。这个动作在口腔深处，所以无法直接看到。绝大多数人其实并不知道自己具体是怎么发出这两个声音的，也没有人能想起来自己小时候是如何学会的。
+首先，这两个音需要舌背后部抬起来顶住软腭而后开始发音。这个动作在口腔深处，所以无法直接看到。绝大多数人其实并不知道自己具体是怎么发出这两个声音的，也没有人能想起来自己小时候是如何学会的。
 
 Firstly, to pronounce these two sounds, the back part of the tongue must be raised and pressed against the *soft palate*. This action takes place deep in the mouth, so it's not something we can see directly. In fact, most people don't even know exactly how they produce these sounds, nor can anyone recall how they learned to make them as children.