Remove the grave accent (`) from the default word regex

Catching typos that contain a grave accent (`) is a niche. I bet we miss
more typos in Markdown files than we detect typos containing a grave
accent (`).

Additionally, this restores compliance with the documentation:
	By default any alphanumeric character, the
	underscore, the hyphen, and the apostrophe is
	used to build words. This option cannot be
	specified together with --write-changes."

Use plural in the above help message:
	is → are

codespell_lib/_codespell.py

@@ -28,7 +28,7 @@
 # autogenerated by setuptools_scm
 from ._version import __version__ as VERSION  # type: ignore  # noqa: N812
 
-word_regex_def = "[\\w\\-'’`]+"
+word_regex_def = r"[\w\-'’]+"
 # While we want to treat characters like ( or " as okay for a starting break,
 # these may occur unescaped in URIs, and so we are more restrictive on the
 # endpoint.  Emails are more restrictive, so the endpoint remains flexible.
@@ -402,7 +402,7 @@ def parse_options(
         type=str,
         help="regular expression that is used to find words. "
         "By default any alphanumeric character, the "
-        "underscore, the hyphen, and the apostrophe is "
+        "underscore, the hyphen, and the apostrophe are "
         "used to build words. This option cannot be "
         "specified together with --write-changes.",
     )

codespell_lib/data/dictionary.txt

@@ -12234,7 +12234,7 @@ cought->caught, cough, fought,
 coul->could
 could'nt->couldn't
 could't->couldn't
-coulden`t->couldn't
+coulden't->couldn't
 couldent->couldn't
 couldn->could, couldn't,
 couldn;t->couldn't
@@ -48160,7 +48160,7 @@ woudl->would
 woudn't->wouldn't
 would'nt->wouldn't
 would't->wouldn't
-woulden`t->wouldn't
+woulden't->wouldn't
 wouldent->wouldn't
 wouldn;t->wouldn't
 wouldnt'->wouldn't