Fix a typo and template links (#418)

`CONTRIBUTING.md` link with relative path is not accessible from
PR page.

Author: MartinP7r

.github/PULL_REQUEST_TEMPLATE.md

@@ -17,5 +17,5 @@ Replace this paragraph with a description of your changes and rationale. Provide
 ### Checklist
 - [ ] I've added at least one test that validates that my change is working, if appropriate
 - [ ] I've followed the code style of the rest of the project
-- [ ] I've read the [Contribution Guidelines](CONTRIBUTING.md)
+- [ ] I've read the [Contribution Guidelines](https://github.com/apple/swift-argument-parser/blob/main/CONTRIBUTING.md)
 - [ ] I've updated the documentation if necessary

.github/PULL_REQUEST_TEMPLATE/NEW.md

@@ -32,5 +32,5 @@ What is the impact of this change on existing users? Does it deprecate or remove
 ### Checklist
 - [ ] I've added at least one test that validates that my change is working, if appropriate
 - [ ] I've followed the code style of the rest of the project
-- [ ] I've read the [Contribution Guidelines](CONTRIBUTING.md)
+- [ ] I've read the [Contribution Guidelines](https://github.com/apple/swift-argument-parser/blob/main/CONTRIBUTING.md)
 - [ ] I've updated the documentation if necessary

Sources/ArgumentParser/Parsable Properties/Errors.swift

@@ -43,7 +43,7 @@ public struct ValidationError: Error, CustomStringConvertible {
 
 /// An error type that only includes an exit code.
 ///
-/// If you're printing custom errors messages yourself, you can throw this error
+/// If you're printing custom error messages yourself, you can throw this error
 /// to specify the exit code without adding any additional output to standard
 /// out or standard error.
 public struct ExitCode: Error, RawRepresentable, Hashable {