doc: fix formatting typo

Signed-off-by: inge4pres <fgualazzi@gmail.com>

Author: inge4pres

README.md

@@ -8,7 +8,7 @@
 **[Installation](#installation)   •  **
 **[Features](#features)   •  **
 **[Examples](#examples)   •  **
-**[Contributing](#contributing)   •  ****
+**[Contributing](#contributing)   •  **
 **[Community](#join-the-community)**
 
 [Zig docs]: https://zig-o11y.github.io/opentelemetry-sdk/