Fix variable name in README.md example code

[r-a-i]

In response to issue bogde#10 this commit fixes the name of a variable "scale" into "loadcell" in the example for the non-blocking mode which originally read:

// 4. Acquire reading without blocking
if (scale.wait_ready_timeout(1000)) {...

Where it should say:

// 4. Acquire reading without blocking
if (loadcell.wait_ready_timeout(1000)) {...
for consistency with the definition at the start of the example

HX711 loadcell;