Add apostrophe to inanimate possessive object (#506)

Lines 78 - 80 of CONTRIBUTING.md state: 

 Use apostrophes of possession,
    [including for inanimate objects](https://english.stackexchange.com/questions/1031/is-using-the-possessive-s-correct-in-the-cars-antenna).
    Eg 'the account's balance' is correct.

CONTRIBUTING.md

@@ -84,7 +84,7 @@ In particular:
   have `instruction handlers` that process `instructions`. Do not refer to
   [instruction handlers](https://solana.com/docs/terminology#instruction-handler)
   as instructions! The reason is simple: an instruction cannot process an
-  instruction. The `multiple` template in Anchor also calls the functions
+  instruction. The `multiple` template in Anchor also calls the function's
   `handler`.
 
 ### Code