Update sed.md (#68)

correct error

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*
Matches a sequence of zero or more instances of matches for the preceding regular expression, which must be an ordinary character, a special character preceded by \, a ., a grouped regexp (see below), or a bracket expression. As a GNU extension, a postfixed regular expression can also be followed by *; for example, a** is equivalent to a*. POSIX 1003.1-2001 says that * stands for itself when it appears at the start of a regular expression or subexpression, but many nonGNU implementations do not support this and portable scripts should instead use \* in these contexts.

ref: https://www.gnu.org/software/sed/manual/sed.html

command/sed.md

@@ -78,7 +78,7 @@ y # 表示把一个字符翻译为另外的字符（但是不用于正则表达
 ^ # 匹配行开始，如：/^sed/匹配所有以sed开头的行。
 $ # 匹配行结束，如：/sed$/匹配所有以sed结尾的行。
 . # 匹配一个非换行符的任意字符，如：/s.d/匹配s后接一个任意字符，最后是d。
-** # 匹配0个或多个字符，如：/*sed/匹配所有模板是一个或多个空格后紧跟sed的行。
+* # 匹配0个或多个字符，如：/*sed/匹配所有模板是一个或多个空格后紧跟sed的行。
 [] # 匹配一个指定范围内的字符，如/[ss]ed/匹配sed和Sed。  
 [^] # 匹配一个不在指定范围内的字符，如：/[^A-RT-Z]ed/匹配不包含A-R和T-Z的一个字母开头，紧跟ed的行。
 \(..\) # 匹配子串，保存匹配的字符，如s/\(love\)able/\1rs，loveable被替换成lovers。