Fixed typo.

Author: ambaxter

book/src/using-bitfields.md

@@ -4,7 +4,7 @@
 
 As Rust does not support bitfields, Bindgen generates a struct for each with the following characteristics
 * Immutable getter functions for each bitfield named ```<bitfield>```
-* Setter functions for each contiguous bsock of bitfields named ```set_<bitfield>```
+* Setter functions for each contiguous block of bitfields named ```set_<bitfield>```
 * Far each contiguous block of bitfields, Bindgen emits an opaque physical field that contains one or more logical bitfields
 * A static constructor  ```new_bitfield_{1, 2, ...}``` with a parameter for each bitfield contained within the opaque physical field.