sensors: Fix some typos in the Blink side's documentation

The platform-specific side is now in services/device/generic_sensor. While
here, fix a small typo ("intrefaces" -> "interfaces").

TBR=alexander.shalamov@intel.com

Change-Id: I810dce6ba28ad9fc6a5b7417431543df8f4b5a5d
Reviewed-on: https://chromium-review.googlesource.com/1093072Reviewed-by: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#565642}

third_party/blink/renderer/modules/sensor/README.md

 # Generic Sensor
 
 `third_party/blink/renderer/modules/sensor` implements the following concrete
-sensor intrefaces based on the [Generic Sensor API]
+sensor interfaces based on the [Generic Sensor API]
 (https://w3c.github.io/sensors):
 1. [Ambient Light Sensor] (https://w3c.github.io/ambient-light)
 1. [Accelerometer] (https://w3c.github.io/accelerometer)
@@ -10,7 +10,7 @@ sensor intrefaces based on the [Generic Sensor API]
 1. [Absolute Orientation Sensor] (https://w3c.github.io/orientation-sensor)
 
 The platform-specific parts of the implementation are located in
-`device/generic_sensor`.
+`services/device/generic_sensor`.
 
 
 ## Testing