Update full-stack-solana-development.md (#566)

content/guides/getstarted/full-stack-solana-development.md

@@ -489,7 +489,7 @@ pub struct Counter {
 
 Make sure you go over the comments!
 
-The `initialize` instruction instruction does only one this: it creates a new
+The `initialize` instruction does only one thing: it creates a new
 account of the `Counter` type. To do this, we need to know who's paying, details
 of the account we're creating like the space and the address, and which program
 to use to create the account.