linux-next: ftrace/docs: Fix spelling typos in ftrace-users.rst

Introduction

============

The ftrace infrastructure was originially created to attach callbacks to the

The ftrace infrastructure was originally created to attach callbacks to the

beginning of functions in order to record and trace the flow of the kernel.

But callbacks to the start of a function can have other use cases. Either

for live kernel patching, or for security monitoring. This document describes

  This requires extra care to what can be done inside a callback. A callback

  can be called outside the protective scope of RCU.

The ftrace infrastructure has some protections agains recursions and RCU

The ftrace infrastructure has some protections against recursions and RCU

but one must still be very careful how they use the callbacks.