Update 0_beacon-chain.md

* Avoid abbreviations
* Use `branch` as a more suggestive variable name than `ret`
* Cleanup spacing after comma
* Avoid having two index variables (`index` and `idx`)—Does this break anything?

specs/core/0_beacon-chain.md

@@ -695,12 +695,12 @@ def get_deposit_root() -> bytes32:
 @public
 @constant
 def get_merkle_branch(index: uint256) -> bytes32[32]: # size is DEPOSIT_CONTRACT_TREE_DEPTH (symbolic const not supported)
-    idx: uint256 = index + TWO_TO_POWER_OF_TREE_DEPTH
-    ret: bytes32[32] # size is DEPOSIT_CONTRACT_TREE_DEPTH
+    index = index + TWO_TO_POWER_OF_TREE_DEPTH
+    branch: bytes32[32] # size is DEPOSIT_CONTRACT_TREE_DEPTH
     for i in range(DEPOSIT_CONTRACT_TREE_DEPTH):
-        ret[i] = self.deposit_tree[bitwise_xor(idx,1)]
-        idx /= 2
-    return ret
+        branch[i] = self.deposit_tree[bitwise_xor(index, 1)]
+        index /= 2
+    return branch
 ```
 
 ## Beacon chain processing