fix documentation typo
`book/src/cpp.md` can have little a typo fix, as a treat.
# Generating Bindings to C++
-`bindgen` can handle a some C++ features, but not all of them. To set
+`bindgen` can handle some C++ features, but not all of them. To set
expectations: `bindgen` will give you the type definitions and FFI declarations
you need to build an API to the C++ library, but using those types in Rust will
be nowhere near as nice as using them in C++. You will have to manually call
