Updated CONTRIBUTING.md: Fixed spelling of 'upstream'. (telegramdesktop#5206)

Author: 23rd

.github/CONTRIBUTING.md

@@ -33,7 +33,7 @@ environments.
 
 ## Pull upstream changes into your fork regularly
 
-Telegram Desktop is advancing quickly. It is therefore critical that you pull upstream changes into your fork on a regular basis. Nothing is worse than putting in a days of hard work into a pull request only to have it rejected because it has diverged too far from upstram.
+Telegram Desktop is advancing quickly. It is therefore critical that you pull upstream changes into your fork on a regular basis. Nothing is worse than putting in a days of hard work into a pull request only to have it rejected because it has diverged too far from upstream.
 
 To pull in upstream changes: