Modified the throw context example with concrete types but not concrete values.

[ioannad]

Addresses @aheejin's request in a previous review comment: https://github.com/WebAssembly/exception-handling/pull/180#discussion_r955477590

Originally I wrote this using

• val_{i32} = (i32.const 1),
• val_{f32} = (f32.const 2.0), and
• val_{i64} = (i64.const 3),

but the example seemed then really long. To keep the example relatively short I switched to the current version.

[ioannad]

@aheejin is the example clear enough now or do you think it's better to use the concrete values, so (i32.const 1) instead of val_{i32} etc?

[ioannad]

@rossberg incidentally, this also addresses your previous comment about using "let ... be .. ".

[ioannad]

All review comments should be addressed now, pending for approval.