Fix rbenv version prompt

[BarbUk]

Description

Fix a typo in rbenv_version_prompt function.

Motivation and Context

When using rbenv_version_prompt to display ruby version
with rbenv installed, there is an error:

bash: 2.7.2: command not found

87% 2021-01-16 09:24:20 ⌚  |2.7.2| ThinkbUk in ~
○ →

The function define a variable containing the ruby version with rbenv version-name
and use that variable like the rbenv binary.

rbenv commands | grep -q gemset is a valid command when using rbenv gemset.

How Has This Been Tested?

No error after modification.
Using rbenv-gemset display the gemset used.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[NoahGorny]

Well done @BarbUk !