Update README.md (#384)

People who follow this build instruction will actually build the
validator from another repository and source code. So let's fix repo
name here.

<!-- greptile_comment -->

## Greptile Summary

Updates repository name and fixes documentation inconsistencies in
README.md to improve clarity and accuracy.

- Fixed repository name in clone instructions from
`ephemeral-conjunto/ephemeral-validator` to
`magicblock-labs/magicblock-validator`
- Issues link and badge URL are inconsistent
(`magicblock-labs/bolt/issues` vs `ephemeral-validator`)
- Configuration files description has grammatical error ('files is'
should be 'files are')
- Image source URL on line 3 should be verified for accessibility



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<!-- /greptile_comment -->

Co-authored-by: Gabriele Picco <piccogabriele@gmail.com>

Author: sudachen

README.md

@@ -47,9 +47,8 @@ $ rustup component add rustfmt
 ### **2. Download the source code.**
 
 ```bash
-$ git clone https://github.com/magicblock-labs/ephemeral-conjunto.git
-$ git clone https://github.com/magicblock-labs/ephemeral-validator.git
-$ cd ephemeral-validator
+$ git clone https://github.com/magicblock-labs/magicblock-validator.git
+$ cd magicblock-validator
 ```
 
 ### **3. Build.**