[chore] Remove trailing bullet separator in README navigation (open-telemetry#44717)

#### Description

Fixed a formatting issue in the README where there was a trailing bullet
separator (`  •  `) with no link after it in
the navigation section. The navigation section now properly ends after
the "Security" link without an extra bullet point.

#### Link to tracking issue
N/A - Minor documentation formatting fix

#### Testing
No testing required - this is a documentation-only change that removes a
formatting artifact.

#### Documentation
This PR fixes a formatting issue in the main README.md file by removing
a trailing bullet separator in the navigation section (line 36).

Co-authored-by: Satbir Singh <geekysatbir@users.noreply.github.com>

Author: geekysatbir

README.md

@@ -33,7 +33,6 @@
     <a href="https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/observability.md">Observability</a>
     &nbsp;&nbsp;&bull;&nbsp;&nbsp;
     <a href="https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/security-best-practices.md">Security</a>
-    &nbsp;&nbsp;&bull;&nbsp;&nbsp;
   </strong>
 </p>