[Merged by Bors] - [docs, ci] update cargo-contract installation

[ascjones]

No description provided.

[Robbepop]

bors r+

[bors]

try

Build succeeded

• ci/gitlab/gitlab.parity.io

[Robbepop]

bors merge

[bors]

Already running a review

[bors]

Build succeeded

• ci/gitlab/gitlab.parity.io

[bors]

Pull request successfully merged into master.

[codecov-io]

Codecov Report

Merging #371 into master will decrease coverage by 4.75%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
- Coverage   19.63%   14.87%   -4.76%     
==========================================
  Files          22       10      -12     
  Lines        1477      531     -946     
==========================================
- Hits          290       79     -211     
+ Misses       1187      452     -735

Impacted Files	Coverage Δ
primitives/src/byte_utils.rs	0% <0%> (-100%)	⬇️
primitives/src/key.rs	0% <0%> (-70.64%)	⬇️
primitives/src/hash.rs	0% <0%> (ø)	⬆️
lang/macro/src/codegen/testable.rs
lang/macro/src/codegen/cross_calling.rs
lang/macro/src/codegen/abi.rs
lang/macro/src/lib.rs
lang/macro/src/codegen/contract.rs
lang/macro/src/ir/into_hir.rs
core/derive/tests/compile_tests.rs
... and 19 more

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