message: Fixed typo in book
diff: @@ -5,7 +5,7 @@ In order to use argmin, one needs to add both `argmin` and `argmin-math` to `Car
  ```toml
  [dependencies]
  argmin = { version = "0.7" }
  -argmin-math = { version = "0.2", features = ["ndarray_latest-serde,nalgebra_latest-serde"] }
  +argmin-math = { version = "0.2", features = ["ndarray_latest-serde", "nalgebra_latest-serde"] }
  ```
  or, for the current development version:
  @@ -13,7 +13,7 @@ or, for the current development version:
  ```toml
  [dependencies]
  argmin = { git = "https://github.com/argmin-rs/argmin" }
  -argmin-math = { git = "https://github.com/argmin-rs/argmin", features = ["ndarray_latest-serde,nalgebra_latest-serde"] }
  +argmin-math = { git = "https://github.com/argmin-rs/argmin", features = ["ndarray_latest-serde", "nalgebra_latest-serde"] }
  ```
  Via adding `argmin-math` one can choose which math backend should be available (and whether `serde`-support is enabled or not).
  