Docs fix

Reflect lpf.h was renamed to filter.h

docs/book/firmware.md

@@ -31,7 +31,7 @@
 
 * [`vector.h`](https://github.com/okalachev/flix/blob/master/flix/vector.h), [`quaternion.h`](https://github.com/okalachev/flix/blob/master/flix/quaternion.h) — библиотеки векторов и кватернионов.
 * [`pid.h`](https://github.com/okalachev/flix/blob/master/flix/pid.h) — ПИД-регулятор.
-* [`lpf.h`](https://github.com/okalachev/flix/blob/master/flix/lpf.h) — фильтр нижних частот.
+* [`filter.h`](https://github.com/okalachev/flix/blob/master/flix/filter.h) — фильтр нижних частот.
 
 ### Подсистема управления
 

docs/firmware.md

@@ -34,7 +34,7 @@ Utility files:
 
 * [`vector.h`](../flix/vector.h), [`quaternion.h`](../flix/quaternion.h) — vector and quaternion libraries.
 * [`pid.h`](../flix/pid.h) — generic PID controller.
-* [`lpf.h`](../flix/lpf.h) — generic low-pass filter.
+* [`filter.h`](../flix/filter.h) — generic low-pass filter.
 
 ### Control subsystem