Fix grammar and improve clarity in API coexistence explanation

The original sentence was ungrammatical and slightly unclear.
This patch improves the article usage, verb tense, and phrasing
to ensure clarity and correctness.

Author: I-Ying-Tsai

lkmpg.tex

@@ -1841,7 +1841,7 @@ \subsection{Flashing keyboard LEDs}
 \end{code}
 
 Since Linux v4.14, \cpp|timer_setup| is adopted and the kernel step by step converting to \cpp|timer_setup| from \cpp|setup_timer|.
-One of the reasons why API was changed is it need to coexist with the old version interface.
+One of the reasons why the API was changed is that it needed to coexist with the old version of the interface.
 Moreover, the \cpp|timer_setup| was implemented by \cpp|setup_timer| at first.
 \begin{code}
 void timer_setup(struct timer_list *timer,