Fix incorrect links to validator examples (#1239)

8c77664 added the files and the links,
but the links were off by one character and therefore don't work. This
PR fixes them

Author: LunarWatcher

README.md

@@ -1752,9 +1752,9 @@ brief description of each is included here
   Short example of subcommands
 - [validators](https://github.com/CLIUtils/CLI11/blob/main/examples/validators.cpp):
   Example illustrating use of validators
-- [custom validators](https://github.com/CLIUtils/CLI11/blob/main/examples/custom_validators.cpp):
+- [custom validators](https://github.com/CLIUtils/CLI11/blob/main/examples/custom_validator.cpp):
   Example illustrating use of validators
-- [date validators](https://github.com/CLIUtils/CLI11/blob/main/examples/date_validators.cpp):
+- [date validators](https://github.com/CLIUtils/CLI11/blob/main/examples/date_validator.cpp):
   Example illustrating use of validators
 
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