patch 8.0.1809: various typos

Problem:    Various typos.
Solution:   Correct the mistakes, change "cursur" to "cursor". (closes #2887)
diff --git a/src/proto/screen.pro b/src/proto/screen.pro
index f04fda5..f690f93 100644
--- a/src/proto/screen.pro
+++ b/src/proto/screen.pro
@@ -12,7 +12,7 @@
 void update_curbuf(int type);
 int update_screen(int type_arg);
 int conceal_cursor_line(win_T *wp);
-void conceal_check_cursur_line(void);
+void conceal_check_cursor_line(void);
 void update_single_line(win_T *wp, linenr_T lnum);
 void update_debug_sign(buf_T *buf, linenr_T lnum);
 void updateWindow(win_T *wp);