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/alloc.c b/src/alloc.c
index 84a1124..20df46c 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -236,7 +236,7 @@
     // if some blocks are released call malloc again.
     for (;;)
     {
-	// Handle three kind of systems:
+	// Handle three kinds of systems:
 	// 1. No check for available memory: Just return.
 	// 2. Slow check for available memory: call mch_avail_mem() after
 	//    allocating KEEP_ROOM amount of memory.