Update installation snippet to adapt to workspace migration (#160)

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## Pull Request type

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- [ ] Bugfix
- [ ] Feature
- [ ] Code style update (formatting, renaming)
- [ ] Refactoring (no functional changes, no API changes)
- [ ] Build-related changes
- [x] Documentation content changes
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## What is the current behavior?

Installation instructions in readme do not work after #154.

## What is the new behavior?

They work now :)

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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README.md

@@ -76,11 +76,12 @@ This repository is divided in 8 modules:
 
 ### Installation
 
-Alexandria is a collection of cairo utilities, which can be installed by adding the following line to your `Scarb.toml`:
+Alexandria is a collection of utility Cairo packages, which have to be added as dependencies separately.
+For example, the `alexandria_math` package can be installed by adding the following line to your `Scarb.toml`:
 
 ```toml
 [dependencies]
-alexandria = { git = "https://github.com/keep-starknet-strange/alexandria.git" }
+alexandria_math = { git = "https://github.com/keep-starknet-strange/alexandria.git" }
 ```
 
 then add the following line in your `.cairo` file