patch 8.0.0766: option test fails with +terminal feature

Problem:    Option test fails with +terminal feature.
Solution:   Fix using the right option when checking the value.
diff --git a/src/option.c b/src/option.c
index 987c5b5..e2dfa88 100644
--- a/src/option.c
+++ b/src/option.c
@@ -7465,7 +7465,7 @@
 
 #ifdef FEAT_TERMINAL
     /* 'termkey' */
-    else if (varp == &curwin->w_p_tms)
+    else if (varp == &curwin->w_p_tk)
     {
 	if (*curwin->w_p_tk != NUL && string_to_key(curwin->w_p_tk, TRUE) == 0)
 	    errmsg = e_invarg;