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/strings.c b/src/strings.c
index 3dd6856..52cf121 100644
--- a/src/strings.c
+++ b/src/strings.c
@@ -171,7 +171,7 @@
     // itself must be escaped to get a literal '\'.
     fish_like = fish_like_shell();
 
-    // PowerShell uses it's own version for quoting single quotes
+    // PowerShell uses its own version for quoting single quotes
     shname = gettail(p_sh);
     powershell = strstr((char *)shname, "pwsh") != NULL;
 # ifdef MSWIN