patch 8.1.2058: function for ex command is named inconsistently

Problem:    Function for ex command is named inconsistently.
Solution:   Rename do_marks() to ex_marks().
diff --git a/src/mark.c b/src/mark.c
index 8a18040..a77e7b9 100644
--- a/src/mark.c
+++ b/src/mark.c
@@ -699,7 +699,7 @@
  * print the marks
  */
     void
-do_marks(exarg_T *eap)
+ex_marks(exarg_T *eap)
 {
     char_u	*arg = eap->arg;
     int		i;