Fix formatting in storage information section

Correct formatting of storage information in README.

Author: githubflyideas

README.md

@@ -21,7 +21,7 @@ tar -zxvf pingping-v3.2.0-linux-amd64.tar.gz
 ```
 Open http://localhost:8517 and watch your first puff of network smoke　　
 
-*v2.x:Storage is JSON ;   history is kept for 300 days by default   
+*v2.x:Storage is JSON ;    history is kept for 300 days by default   
 *v3.x:Storage is SQLite; history is kept for 40  days by default (--days=300 for longer)
 
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