patch 8.2.1436: function implementing :substitute has unexpected name

Problem:    Function implementing :substitute has unexpected name.
Solution:   Rename from do_sub() to ex_substitute().

src/ex_cmds.c

@@ -3553,7 +3553,7 @@ typedef struct {
  * The usual escapes are supported as described in the regexp docs.
  */
     void
-do_sub(exarg_T *eap)
+ex_substitute(exarg_T *eap)
 {
     linenr_T	lnum;
     long	i = 0;