[3.8] Fix typos in docs, comments and test assert messages (GH-14872). (#14900)

(cherry picked from commit 96e12d5f4f3c5a20986566038ee763dff3c228a1)

Co-authored-by: Min ho Kim <minho42@gmail.com>
diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 1871372..23712fa 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -1189,7 +1189,7 @@
             raise TypeError(f'Invalid field: {item!r}')
 
         if not isinstance(name, str) or not name.isidentifier():
-            raise TypeError(f'Field names must be valid identifers: {name!r}')
+            raise TypeError(f'Field names must be valid identifiers: {name!r}')
         if keyword.iskeyword(name):
             raise TypeError(f'Field names must not be keywords: {name!r}')
         if name in seen: