Fix env::block_timestamp example

[cmichi]

I noticed that the example contains a mistake. The comment says

/// The function can be executed at most once every 100 blocks.

But the env function used there is actually block_timestamp().

I first thought about just modifying the comment to

/// The function can be executed at most once every 100 milliseconds.

But since we don't know if the environment Timestamp actually counts in milliseconds we can't do that.

[codecov-commenter]

Codecov Report

Merging #937 (21426e8) into master (882b4dd) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #937      +/-   ##
==========================================
- Coverage   82.84%   82.76%   -0.08%     
==========================================
  Files         177      177              
  Lines        8066     8066              
==========================================
- Hits         6682     6676       -6     
- Misses       1384     1390       +6     

Impacted Files	Coverage Δ
crates/lang/src/env_access.rs	27.27% <ø> (ø)
...ates/storage/src/collections/hashmap/fuzz_tests.rs	93.61% <0.00%> (-6.39%)	⬇️

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[Robbepop]

LGTM