fix(derive): Match rustc's error style

This was missed in #4602

Author: epage

clap_derive/src/derives/args.rs

@@ -735,13 +735,13 @@ fn gen_parsers(
                 Name::Assigned(_) => {
                     quote_spanned! { ty.span()=>
                         #arg_matches.#get_one(#id)
-                            .ok_or_else(|| clap::Error::raw(clap::error::ErrorKind::MissingRequiredArgument, format!("The following required argument was not provided: {}", #id)))?
+                            .ok_or_else(|| clap::Error::raw(clap::error::ErrorKind::MissingRequiredArgument, format!("the following required argument was not provided: {}", #id)))?
                     }
                 }
                 Name::Derived(_) => {
                     quote_spanned! { ty.span()=>
                         #arg_matches.#get_one(#id)
-                            .ok_or_else(|| clap::Error::raw(clap::error::ErrorKind::MissingRequiredArgument, concat!("The following required argument was not provided: ", #id)))?
+                            .ok_or_else(|| clap::Error::raw(clap::error::ErrorKind::MissingRequiredArgument, concat!("the following required argument was not provided: ", #id)))?
                     }
                 }
             }

clap_derive/src/derives/subcommand.rs

@@ -548,7 +548,7 @@ fn gen_from_arg_matches(variants: &[(&Variant, Item)]) -> Result<TokenStream, sy
         },
 
         None => quote! {
-            ::std::result::Result::Err(clap::Error::raw(clap::error::ErrorKind::InvalidSubcommand, format!("The subcommand '{}' wasn't recognized", #subcommand_name_var)))
+            ::std::result::Result::Err(clap::Error::raw(clap::error::ErrorKind::InvalidSubcommand, format!("the subcommand '{}' wasn't recognized", #subcommand_name_var)))
         },
     };
 
@@ -565,7 +565,7 @@ fn gen_from_arg_matches(variants: &[(&Variant, Item)]) -> Result<TokenStream, sy
 
                 #wildcard
             } else {
-                ::std::result::Result::Err(clap::Error::raw(clap::error::ErrorKind::MissingSubcommand, "A subcommand is required but one was not provided."))
+                ::std::result::Result::Err(clap::Error::raw(clap::error::ErrorKind::MissingSubcommand, "a subcommand is required but one was not provided"))
             }
         }
     })

examples/derive_ref/interop_tests.md

@@ -48,7 +48,7 @@ For more information, try '--help'.
 ```console
 $ interop_augment_subcommands
 ? failed
-error: A subcommand is required but one was not provided.
+error: a subcommand is required but one was not provided
 ```
 
 ```console

tests/derive/help.rs

@@ -221,7 +221,7 @@ fn derive_generated_error_has_full_context() {
     );
 
     let expected = str![[r#"
-error: The following required argument was not provided: req_str
+error: the following required argument was not provided: req_str
 
 Usage: clap --req-str <REQ_STR>
        clap <COMMAND>