Fix grammar and improve clarity in API coexistence explanation

Original:
    "One of the reasons why API was changed is it
     need to coexist with the old version interface."

Issues:
- Missing article ("the API")
- Verb tense disagreement ("it need" → "it needed")
- Unnatural phrase ("old version interface")

Revised:
    "One of the reasons why the API was changed is that
     it needed to coexist with the old version of the interface."

This change improves grammar, consistency,
and readability in the explanation.

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,