Update README.md to match example code for POST
Update README.md to match example code for POST

The "Post to a Server" section in the README is using the deprecated
method for creating a new `RequestBody`. This commit uses the
recommended method for the README, but also now matches the example in
`PostExample.java`

square/okhttp,yschimke/okhttp,tiarebalbi/okhttp,square/okhttp,ansman/okhttp,ze-pequeno/okhttp,cmzy/okhttp,square/okhttp,cketti/okhttp,tiarebalbi/okhttp,ze-pequeno/okhttp,ze-pequeno/okhttp,tiarebalbi/okhttp,cmzy/okhttp,yschimke/okhttp,ansman/okhttp,yschimke/okhttp,ansman/okhttp,cketti/okhttp,cketti/okhttp