Update install-linux-manual.rst

[ehharris]

Fix slight typo

Requirements

• [y] Content follows Splunk guidelines for style and formatting.
• [y] You are contributing original content.

Describe the change
Hello, I was testing installing this and found that I had to make a small correction to make it work. Let me know if you have any questions

[aurbiztondo-splunk]

Thanks for the fix, @ehharris!