Delete duplicate does in 3 design examples (#24)

Delete duplicate word "does" in design exercises

solutions/system_design/pastebin/README.md

@@ -95,7 +95,7 @@ An alternative to a relational database acting as a large hash table, we could u
 
 * The **Client** sends a create paste request to the **Web Server**, running as a [reverse proxy](https://github.com/donnemartin/system-design-primer#reverse-proxy-web-server)
 * The **Web Server** forwards the request to the **Write API** server
-* The **Write API** server does does the following:
+* The **Write API** server does the following:
     * Generates a unique url
         * Checks if the url is unique by looking at the **SQL Database** for a duplicate
         * If the url is not unique, it generates another url

solutions/system_design/social_graph/README.md

@@ -104,7 +104,7 @@ We won't be able to fit all users on the same machine, we'll need to [shard](htt
 * The **Client** sends a request to the **Web Server**, running as a [reverse proxy](https://github.com/donnemartin/system-design-primer#reverse-proxy-web-server)
 * The **Web Server** forwards the request to the **Search API** server
 * The **Search API** server forwards the request to the **User Graph Service**
-* The **User Graph Service** does does the following:
+* The **User Graph Service** does the following:
     * Uses the **Lookup Service** to find the **Person Server** where the current user's info is stored
     * Finds the appropriate **Person Server** to retrieve the current user's list of `friend_ids`
     * Runs a BFS search using the current user as the `source` and the current user's `friend_ids` as the ids for each `adjacent_node`

solutions/system_design/web_crawler/README.md

@@ -213,7 +213,7 @@ We might also choose to support a `Robots.txt` file that gives webmasters contro
 
 * The **Client** sends a request to the **Web Server**, running as a [reverse proxy](https://github.com/donnemartin/system-design-primer#reverse-proxy-web-server)
 * The **Web Server** forwards the request to the **Query API** server
-* The **Query API** server does does the following:
+* The **Query API** server does the following:
     * Parses the query
         * Removes markup
         * Breaks up the text into terms