patch 8.2.3914: various spelling mistakes in comments

Problem:    Various spelling mistakes in comments.
Solution:   Fix the mistakes. (Dominique Pellé, closes #9416)
diff --git a/src/quickfix.c b/src/quickfix.c
index d0f4199..c007a9c 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -4537,7 +4537,7 @@
     int		len;
     buf_T	*errbuf;
 
-    // If the 'quickfixtextfunc' function returned an non-empty custom string
+    // If the 'quickfixtextfunc' function returned a non-empty custom string
     // for this entry, then use it.
     if (qftf_str != NULL && *qftf_str != NUL)
 	vim_strncpy(IObuff, qftf_str, IOSIZE - 1);