chore: Update README.md (#1470)

Grammar fix in doc's introduction.

README.md

@@ -5,7 +5,7 @@
 Fast HTTP implementation for Go.
 
 # fasthttp might not be for you!
-fasthttp was design for some high performance edge cases. **Unless** your server/client needs to handle **thousands of small to medium requests per seconds** and needs a consistent low millisecond response time fasthttp might not be for you. **For most cases `net/http` is much better** as it's easier to use and can handle more cases. For most cases you won't even notice the performance difference.
+fasthttp was designed for some high performance edge cases. **Unless** your server/client needs to handle **thousands of small to medium requests per second** and needs a consistent low millisecond response time fasthttp might not be for you. **For most cases `net/http` is much better** as it's easier to use and can handle more cases. For most cases you won't even notice the performance difference.
 
 
 ## General info and links