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/2.1.5-ə.md

@@ -12,7 +12,7 @@ Beginners tend to overdo it and add a <span class="pho">r</span> whenever they e
 
 The rhotacized <span class="pho">ɚ</span> and <span class="pho">ɝː</span> sounds, can be produced in two different ways. Most beginners instinctively curl their tongue tip to produce this sound, as it's generally the only method they're familiar with.
 
-由于这个音发声时的舌头动作是别人看不见的，所以，人们在学这个发音的时候，无法猜到第二种方式：舍尾部分抬起来，使舌的两侧接触到后牙。舌尾的中心部分较低，空气通过这个凹槽产生声音。
+由于这个音发声时的舌头动作是别人看不见的，所以，人们在学这个发音的时候，无法猜到第二种方式：舌尾部分抬起来，使舌的两侧接触到后牙。舌尾的中心部分较低，空气通过这个凹槽产生声音。
 
 ![articulator-tongue-tip-ɚ](/images/articulator-tongue-tip-ɚ.svg)