test: Use POSIX-compliant example for equals comparison (#11728)

* [: use posix-compliant example for equals comparison

the previous example worked fine for bash, but some
other shells (zsh, in my case) will not work when
using "==" for comparison. the posix spec only requires
"=", so I think it makes a little more sense to use
that in the example.

* test: use posix-compliant example for equals comparison

the previous example worked fine for bash, but some
other shells (zsh, in my case) will not work when
using "==" for comparison. the posix spec only requires
"=", so I think it makes a little more sense to use
that in the example.

pages.zh/common/[.md

@@ -6,7 +6,7 @@
 
 - 测试一个给定的变量是否等于/不等于指定的字符串：
 
-`[ "${{变量}}" {{==|!=}} "{{字符串}}" ]`
+`[ "${{变量}}" {{=|!=}} "{{字符串}}" ]`
 
 - 测试一个给定的变量是否等于/不等于/大于/小于/大于等于/小于等于指定的数字：