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/ex_cmds.h b/src/ex_cmds.h
index 2490b54..09d62a9 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -928,7 +928,7 @@
 EXCMD(CMD_mapclear,	"mapclear",	ex_mapclear,
 	EX_EXTRA|EX_BANG|EX_TRLBAR|EX_CMDWIN,
 	ADDR_NONE),
-EXCMD(CMD_marks,	"marks",	do_marks,
+EXCMD(CMD_marks,	"marks",	ex_marks,
 	EX_EXTRA|EX_TRLBAR|EX_CMDWIN,
 	ADDR_NONE),
 EXCMD(CMD_match,	"match",	ex_match,