docs: add severity example (#350)

Co-authored-by: hzlinyiyu <hzlinyiyu@corp.netease.com>

README.md

@@ -425,7 +425,7 @@ pub struct MyErrorType {
 }
 ```
 
-##### ... help text
+#### ... help text
 `miette` provides two facilities for supplying help text for your errors:
 
 The first is the `#[help()]` format attribute that applies to structs or
@@ -461,6 +461,19 @@ let err = Foo {
 };
 ```
 
+#### ... severity level
+`miette` provides a way to set the severity level of a diagnostic.
+
+```rust
+use miette::Diagnostic;
+use thiserror::Error;
+
+#[derive(Debug, Diagnostic, Error)]
+#[error("welp")]
+#[diagnostic(severity(Warning))]
+struct Foo;
+```
+
 #### ... multiple related errors
 
 `miette` supports collecting multiple errors into a single diagnostic, and
@@ -635,7 +648,7 @@ then you may want to use [`miette!`], [`diagnostic!`] macros or
 
 let source = "2 + 2 * 2 = 8".to_string();
 let report = miette!(
-  labels = vec[
+  labels = vec![
       LabeledSpan::at(12..13, "this should be 6"),
   ],
   help = "'*' has greater precedence than '+'",