Fixed typos in README.md (#207)

Author: jecs

README.md

@@ -17,11 +17,11 @@ See also the Python package [FDM](https://github.com/wesselb/fdm).
 #### FiniteDiff.jl vs FiniteDifferences.jl
 [FiniteDiff.jl](https://github.com/JuliaDiff/FiniteDiff.jl) and [FiniteDifferences.jl](https://github.com/JuliaDiff/FiniteDifferences.jl)
 are similar libraries: both calculate approximate derivatives numerically.
-You should definately use one or the other, rather than the legacy [Calculus.jl](https://github.com/JuliaMath/Calculus.jl) finite differencing, or reimplementing it yourself.
+You should definitely use one or the other, rather than the legacy [Calculus.jl](https://github.com/JuliaMath/Calculus.jl) finite differencing, or reimplementing it yourself.
 At some point in the future they might merge, or one might depend on the other.
 Right now here are the differences:
 
- - FiniteDifferences.jl supports basically any type, where as FiniteDiff.jl supports only array-ish types
+ - FiniteDifferences.jl supports basically any type, whereas FiniteDiff.jl supports only array-ish types
  - FiniteDifferences.jl supports higher order approximation and step size adaptation
  - FiniteDiff.jl supports caching and in-place computation
  - FiniteDiff.jl supports coloring vectors for efficient calculation of sparse Jacobians
@@ -81,7 +81,7 @@ julia> central_fdm(12, 2)(sin, 1) - (-sin(1))
 ```
 
 The functions `forward_fdm` and `backward_fdm` can be used to construct
-forward differences and backward differences respectively.
+forward differences and backward differences, respectively.
 
 ### Dealing with Singularities