docs(cookbook): Hint at overriding value_name

Author: epage

examples/typed-derive/fn_parser.md

@@ -4,8 +4,8 @@ $ typed-derive fn-parser --help
 Usage: typed-derive fn-parser [OPTIONS]
 
 Options:
-  -D <DEFINES>      Hand-written parser for tuples
-  -h, --help        Print help
+  -D <KEY=VALUE>      Hand-written parser for tuples
+  -h, --help          Print help
 
 ```
 
@@ -16,13 +16,13 @@ FnParser(FnParser { defines: [("Foo", 10), ("Alice", 30)] })
 
 $ typed-derive fn-parser -D Foo
 ? failed
-error: invalid value 'Foo' for '-D <DEFINES>': invalid KEY=value: no `=` found in `Foo`
+error: invalid value 'Foo' for '-D <KEY=VALUE>': invalid KEY=value: no `=` found in `Foo`
 
 For more information, try '--help'.
 
 $ typed-derive fn-parser -D Foo=Bar
 ? failed
-error: invalid value 'Foo=Bar' for '-D <DEFINES>': invalid digit found in string
+error: invalid value 'Foo=Bar' for '-D <KEY=VALUE>': invalid digit found in string
 
 For more information, try '--help'.
 

examples/typed-derive/fn_parser.rs

@@ -5,7 +5,7 @@ use clap::Args;
 #[derive(Args, Debug)]
 pub(crate) struct FnParser {
     /// Hand-written parser for tuples
-    #[arg(short = 'D', value_parser = parse_key_val::<String, i32>)]
+    #[arg(short = 'D', value_name = "KEY=VALUE", value_parser = parse_key_val::<String, i32>)]
     defines: Vec<(String, i32)>,
 }