Fix usage example

Author: patrickslarson8

README.rst

@@ -67,7 +67,7 @@ Usage Example
     import adafruit_sgp40
 
     i2c = board.I2C()  # uses board.SCL and board.SDA
-    sgp = adafruit_sgp40(i2c)
+    sgp = adafruit_sgp40.SGP40(i2c)
 
     while True:
         print("Measurement: ", sgp.raw)