Fix typos (#417)

Author: kianmeng

CHANGELOG.md

@@ -90,7 +90,7 @@ with httpcore.SyncConnectionPool() as http:
 
 ### Fixed
 
-- Task cancelation no longer leaks connections from the connection pool. (Pull #305)
+- Task cancellation no longer leaks connections from the connection pool. (Pull #305)
 
 ## 0.12.3 (December 7th, 2020)
 

docs/contributing.md

@@ -84,7 +84,7 @@ as part of the unasync process.
 - Run `scripts/unasync` to generate the synchronous versions. Note the script
 is ran as part of other scripts as well, so you don't usually need to run this
 yourself.
-- Run the entire test suite as decribed below.
+- Run the entire test suite as described below.
 
 ## Testing and Linting