Fix `env::block_timestamp` example (#937)

* Fix `env::block_timestamp()` example

* Add clarifying note for `env::block_timestamp()`

* Add example for `env::block_number()`
22 jobs for master in 34 minutes and 29 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Check
passed #1136489
linux-docker
check-std

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passed #1136490
linux-docker
check-wasm

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  Workspace
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linux-docker
build-std

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passed #1136492
linux-docker
build-wasm

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linux-docker
clippy-std

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passed #1136498
linux-docker
clippy-wasm

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linux-docker
codecov

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linux-docker
docs

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linux-docker
fmt

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linux-docker
spellcheck

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passed #1136493
linux-docker
test

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  Examples
passed #1136503
linux-docker
examples-clippy-std

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passed #1136504
linux-docker
examples-clippy-wasm

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passed #1136505
linux-docker
examples-contract-build

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passed #1136506
linux-docker
examples-contract-build-delegator

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passed #1136507
linux-docker
examples-docs

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passed #1136502
linux-docker
examples-fmt

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linux-docker
examples-test

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linux-docker
examples-test-experimental-engine

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  Ink Waterfall
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kubernetes-parity-build
ink-waterfall

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  Fuzz
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linux-docker
fuzz-tests

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  Publish
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kubernetes-parity-build
publish-docs

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