BE-178: changed the php name for the composer package

#### Summary:

This PR contains changes in the documentation related to composer PHP-SDK package.
#### Intended Effect:

Now when the visitors check and copy the composer.json example for installing the PHP-SDK package and use it to install the package the correct package will be downloaded and installed.
#### How to Verify:

Test by checking this PR https://github.com/beepsend/docs/pull/65.
#### Side Effects:

There should be no side effects.
#### Reviewer Suggestions:

https://beepsend.atlassian.net/browse/BE-178