Possible typo in docs (#4088)

I think the author missed a "this" in the phrase

Author: blackblather

docs/callbacks-promises-events.rst

@@ -9,7 +9,7 @@ To help web3 integrate into all kinds of projects with different standards we pr
 Most web3.js objects allow a callback as the last parameter, as well as returning promises to chain functions.
 
 Ethereum as a blockchain has different levels of finality and therefore needs to return multiple "stages" of an action.
-To cope with requirement we return a "promiEvent" for functions like ``web3.eth.sendTransaction`` or contract methods.
+To cope with this requirement we return a "promiEvent" for functions like ``web3.eth.sendTransaction`` or contract methods.
 This "promiEvent" is a promise combined with an event emitter to allow acting on different stages of action on the blockchain, like a transaction.
 
 PromiEvents work like a normal promises with added ``on``, ``once`` and ``off`` functions.