use regex_match in constraint docs (#1953)

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> [!IMPORTANT]
> Replaces `contains` with `regex_match` in `checks-and-asserts.mdx` for
more precise pattern matching in `@assert` and `@check` attributes.
> 
>   - **Behavior**:
> - Replaces `contains` with `regex_match` in `checks-and-asserts.mdx`
for `@assert` and `@check` attributes in `Foo` class example.
> - Specifically changes `bar_no_foo`, `bar_no_fizzle`, and `bar_no_baz`
constraints to use `regex_match` for pattern matching.
> 
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Author: imalsogreg

fern/01-guide/05-baml-advanced/checks-and-asserts.mdx

@@ -303,9 +303,9 @@ You can also chain multiple `@check` and `@assert` attributes on a single field.
 ```baml BAML
 class Foo {
   bar string @check(bar_nonempty, {{ this|length > 0 }})
-  @assert(bar_no_foo, {{ this|contains("foo") }})
-  @check(bar_no_fizzle, {{ this|contains("fizzle") }})
-  @assert(bar_no_baz, {{ this|contains("baz") }})
+  @assert(bar_no_foo, {{ this|regex_match("foo") }})
+  @check(bar_no_fizzle, {{ this|regex_match("fizzle") }})
+  @assert(bar_no_baz, {{ this|regex_match("baz") }})
 }
 ```