docs: update ipi installation command (#4081)

This was addressed in #3835 but the documentation for i-PI wasn't
updated.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Updated the installation command for i-PI software to standardize
package naming.
- **Documentation**
	- Revised installation instructions to reflect the new package name.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Jinzhe Zeng <[email protected]>

doc/install/install-ipi.md

@@ -4,7 +4,7 @@ The i-PI works in a client-server model. The i-PI provides the server for integr
 To use i-PI with already existing drivers, install and update using Pip:
 
 ```bash
-pip install -U i-PI
+pip install -U ipi
 ```
 
 Test with Pytest: