Update 01_1_Introducing_Bitcoin.md

Fixed typo: changed "either if" to "either of".

Author: lvaiser

01_1_Introducing_Bitcoin.md

@@ -80,7 +80,7 @@ Though you need to understand the basics of how a blockchain works to understand
 
 **_Why Is It Called a Chain?_** Each block in the blockchain stores a hash of the block before it. This links the current block all the way back to the original "genesis block" through an unbroken chain. It's a way to create absolute order among possibly conflicting data. This also provides the security of blockchain, because each block is stacked atop an old one makes it harder to recreate the old block due to the proof-of-work algorithms used in block creation. Once several blocks have been built atop a block in the chain, it's essentially irreversible.
 
-**_What is a Fork?_** Occasionally two blocks are created around the same time. This temporarily creates a one-block fork, where either if the current blocks could be the "real" one. Every once in a while, a fork might expand to become two blocks, three blocks, or even four blocks long, but pretty quickly one side of the fork is determined to be the real one, and the other is "orphaned". This is part of the stochastic process of block creation, and demonstrates why several blocks must be built atop a block before it can be considered truly trustworthy and non-repudiable.
+**_What is a Fork?_** Occasionally two blocks are created around the same time. This temporarily creates a one-block fork, where either of the current blocks could be the "real" one. Every once in a while, a fork might expand to become two blocks, three blocks, or even four blocks long, but pretty quickly one side of the fork is determined to be the real one, and the other is "orphaned". This is part of the stochastic process of block creation, and demonstrates why several blocks must be built atop a block before it can be considered truly trustworthy and non-repudiable.
 
 ### Blockchain — In Short