Fix typo.

posts/powf_is_slow.md

@@ -42,7 +42,7 @@ of `libm::powf`.
 
 When compiling with the default optimization level (`-O3`), I saw a similar performance
 increase, but not when I switched to compiling with `--release`, which I found interesting,
-and which underscores the importance of building in `--release` which shipping Rust binaries.
+and which underscores the importance of building in `--release` when shipping Rust binaries.
 
 [libm]: https://github.com/rust-lang/libm
 [micromath]: https://github.com/tarcieri/micromath