Remove redundant SQL index in Pastebin exercise (donnemartin#405)

Author: John-Richardson

solutions/system_design/pastebin/README.md

@@ -116,7 +116,7 @@ paste_path varchar(255) NOT NULL
 PRIMARY KEY(shortlink)
 ```
 
-We'll create an [index](https://github.com/donnemartin/system-design-primer#use-good-indices) on `shortlink ` and `created_at` to speed up lookups (log-time instead of scanning the entire table) and to keep the data in memory. Reading 1 MB sequentially from memory takes about 250 microseconds, while reading from SSD takes 4x and from disk takes 80x longer.<sup><a href=https://github.com/donnemartin/system-design-primer#latency-numbers-every-programmer-should-know>1</a></sup>
+Setting the primary key to be based on the `shortlink` column creates an [index](https://github.com/donnemartin/system-design-primer#use-good-indices) that the database uses to enforce uniqueness. We'll create an additional index on `created_at` to speed up lookups (log-time instead of scanning the entire table) and to keep the data in memory. Reading 1 MB sequentially from memory takes about 250 microseconds, while reading from SSD takes 4x and from disk takes 80x longer.<sup><a href=https://github.com/donnemartin/system-design-primer#latency-numbers-every-programmer-should-know>1</a></sup>
 
 To generate the unique url, we could: