Fixed typo in README.md file. "-regest" "-regtest"

There was a typo in the README.md file. It previously read "-regest" instead of the proper "-regtest" and it has been updated to the proper reading.

README.md

@@ -112,7 +112,7 @@ are testing multi-machine code that needs to operate across the internet.
 
 If you are testing something that can run on one machine, run with the -regtest option.
 In regression test mode, blocks can be created on-demand; see qa/rpc-tests/ for tests
-that run in -regest mode.
+that run in -regtest mode.
 
 **DEBUG_LOCKORDER**