End 'force-exclude' help message with a period (#1727)

It would be nice, if like other options help message, force-exclude's
help message also ends with a period punctuation mark.

README.md

@@ -133,7 +133,7 @@ Options:
 
   --force-exclude TEXT            Like --exclude, but files and directories
                                   matching this regex will be excluded even
-                                  when they are passed explicitly as arguments
+                                  when they are passed explicitly as arguments.
 
   -q, --quiet                     Don't emit non-error messages to stderr.
                                   Errors are still emitted; silence those with

docs/installation_and_usage.md

@@ -88,7 +88,7 @@ Options:
 
   --force-exclude TEXT            Like --exclude, but files and directories
                                   matching this regex will be excluded even
-                                  when they are passed explicitly as arguments
+                                  when they are passed explicitly as arguments.
 
   -q, --quiet                     Don't emit non-error messages to stderr.
                                   Errors are still emitted; silence those with

src/black/__init__.py

@@ -454,7 +454,7 @@ def target_version_option_callback(
     type=str,
     help=(
         "Like --exclude, but files and directories matching this regex will be "
-        "excluded even when they are passed explicitly as arguments"
+        "excluded even when they are passed explicitly as arguments."
     ),
 )
 @click.option(