Fix typos in Twitter and web crawler exercises (#438)

solutions/system_design/twitter/README.md

@@ -26,7 +26,7 @@ Without an interviewer to address clarifying questions, we'll define some use ca
 #### Out of scope
 
 * **Service** pushes tweets to the Twitter Firehose and other streams
-* **Service** strips out tweets based on user's visibility settings
+* **Service** strips out tweets based on users' visibility settings
     * Hide @reply if the user is not also following the person being replied to
     * Respect 'hide retweets' setting
 * Analytics
@@ -129,7 +129,7 @@ If our **Memory Cache** is Redis, we could use a native Redis list with the foll
 | tweet_id  user_id  meta   | tweet_id  user_id  meta   | tweet_id  user_id  meta   |
 ```
 
-The new tweet would be placed in the **Memory Cache**, which populates user's home timeline (activity from people the user is following).
+The new tweet would be placed in the **Memory Cache**, which populates the user's home timeline (activity from people the user is following).
 
 We'll use a public [**REST API**](https://github.com/donnemartin/system-design-primer#representational-state-transfer-rest):
 

solutions/system_design/web_crawler/README.md

@@ -77,7 +77,7 @@ Handy conversion guide:
 
 ### Use case: Service crawls a list of urls
 
-We'll assume we have an initial list of `links_to_crawl` ranked initially based on overall site popularity.  If this is not a reasonable assumption, we can seed the crawler with popular sites that link to outside content such as [Yahoo](https://www.yahoo.com/), [DMOZ](http://www.dmoz.org/), etc
+We'll assume we have an initial list of `links_to_crawl` ranked initially based on overall site popularity.  If this is not a reasonable assumption, we can seed the crawler with popular sites that link to outside content such as [Yahoo](https://www.yahoo.com/), [DMOZ](http://www.dmoz.org/), etc.
 
 We'll use a table `crawled_links` to store processed links and their page signatures.