Fix typo (#86319)

Im working on the floating point fmaximum and fminimum functions right
now in a different pr and I ran the individual tests by doing

```
ninja libc.test.src.math.smoke.<func>_test.__unit__
```

Author: Job Henandez Lara

libc/src/math/docs/add_math_function.md

@@ -177,7 +177,7 @@ implementation (which is very often glibc).
 
 - Build and Run a specific unit test:
 ```
-  $ ninja libc.test.src.math.<func>_test
+  $ ninja libc.test.src.math.<func>_test.__unit__
   $ projects/libc/test/src/math/libc.test.src.math.<func>_test
 ```