[libc] Wrong header guard comment for atanpif16 (#188310)
This PR intends to fix a small nit caused in
[1c1135b](https://github.com/llvm/llvm-project/pull/150400/commits/1c1135b3fccf59537243fc365e83a568f77273ae)
```
#endif // LLVM_LIBC_SRC_MATH_ASINIF16_H
```
to
```
#endif // LLVM_LIBC_SRC_MATH_ATANPIF16_H
```
