[README.md#Mac_OSX_Installation_of_pre-compiled_binaries] Use cURL instead of Wget

[ktgvv]

macOS installation instructions should mention curl instead of wget since Wget isn't installed by default on macOS. 'Novice' users who ended here from Google may not know Homebrew. I suggest you ~~some example of~~ a new command line below, ~~but I haven't tested it.~~

Dropbox's command lines:

$ wget https://github.com/dropbox/dbxcli/releases/download/v2.0.9/dbxcli-darwin-amd64 
$ mv dbxcli-darwin-amd64 dbxcli

My CL:

curl -J -L -H "Accept: application/octet-stream" "https://github.com/dropbox/dbxcli/releases/download/v2.1.0/dbxcli-darwin-amd64" --output "dbxcli"

[diwakergupta]

Sure, feel free to send a PR!