Issue #25523: Correct "a" article to "an" article

This changes the main documentation, doc strings, source code comments, and a
couple error messages in the test suite. In some cases the word was removed
to fix the grammar.
diff --git a/Lib/idlelib/EditorWindow.py b/Lib/idlelib/EditorWindow.py
index 0c5b713..8a33719 100644
--- a/Lib/idlelib/EditorWindow.py
+++ b/Lib/idlelib/EditorWindow.py
@@ -1384,7 +1384,7 @@
             text.see("insert")
             text.undo_block_stop()
 
-    # Our editwin provides a is_char_in_string function that works
+    # Our editwin provides an is_char_in_string function that works
     # with a Tk text index, but PyParse only knows about offsets into
     # a string. This builds a function for PyParse that accepts an
     # offset.
diff --git a/Lib/idlelib/ReplaceDialog.py b/Lib/idlelib/ReplaceDialog.py
index 9a9f74c..66a871a 100644
--- a/Lib/idlelib/ReplaceDialog.py
+++ b/Lib/idlelib/ReplaceDialog.py
@@ -59,7 +59,7 @@
     def default_command(self, event=None):
         if self.do_find(self.ok):
             if self.do_replace():   # Only find next match if replace succeeded.
-                                    # A bad re can cause a it to fail.
+                                    # A bad re can cause it to fail.
                 self.do_find(0)
 
     def _replace_expand(self, m, repl):