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/testdir/test_debugger.vim b/src/testdir/test_debugger.vim
index 6f01917..e305033 100644
--- a/src/testdir/test_debugger.vim
+++ b/src/testdir/test_debugger.vim
@@ -948,7 +948,7 @@
 
   call RunDbgCmd(buf, 'step', ['line 1: SourceAnotherFile()'])
   call RunDbgCmd(buf, 'step', ['line 1: source Xtest2.vim'])
-  " Repeated line, because we fist are in the compiled function before the
+  " Repeated line, because we first are in the compiled function before the
   " EXEC and then in do_cmdline() before the :source command.
   call RunDbgCmd(buf, 'step', ['line 1: source Xtest2.vim'])
   call RunDbgCmd(buf, 'step', ['line 1: vim9script'])