Fix typo.

Author: Alexis Hunt

src/libstd/ffi/mod.rs

@@ -53,7 +53,7 @@
 //! terminator, so the buffer length is really `len+1` characters.
 //! Rust strings don't have a nul terminator; their length is always
 //! stored and does not need to be calculated. While in Rust
-//! accessing a string's length is a O(1) operation (becasue the
+//! accessing a string's length is a O(1) operation (because the
 //! length is stored); in C it is an O(length) operation because the
 //! length needs to be computed by scanning the string for the nul
 //! terminator.