fix typo (#1190)

* 校对: chapter2/03_Strings_and_Characters 方法名变化

index(of:) 已更名为: firstIndex(of:)

* Update 03_Strings_and_Characters.md

fix typo

Author: wtracyliu

source/02_language_guide/03_Strings_and_Characters.md

@@ -89,7 +89,7 @@ It also ends with a line break.
 
 ```swift
 let wiseWords = "\"Imagination is more important than knowledge\" - Einstein"
-// "Imageination is more important than knowledge" - Enistein
+// "Imagination is more important than knowledge" - Einstein
 let dollarSign = "\u{24}"             // $，Unicode 标量 U+0024
 let blackHeart = "\u{2665}"           // ♥，Unicode 标量 U+2665
 let sparklingHeart = "\u{1F496}"      // 💖，Unicode 标量 U+1F496