Fix two typos (#638)

Signed-off-by: Alexander Seiler <[email protected]>

Author: goggle

docs/src/dev/how_it_works.md

@@ -39,7 +39,7 @@ central feature that allows ForwardDiff to take derivatives.
 
 In order to implement the above property, elementary numerical functions on a `Dual`
 number are overloaded to evaluate both the original function, *and* evaluate the derivative
-of the function, propogating the derivative via multiplication. For example, `Base.sin`
+of the function, propagating the derivative via multiplication. For example, `Base.sin`
 can be overloaded on `Dual` like so:
 
 ```julia

docs/src/user/advanced.md

@@ -72,7 +72,7 @@ julia> @time gradient!(out, rosenbrock, x, cfg10);
   0.282529 seconds (4 allocations: 160 bytes)
 ```
 
-If you do not explicity provide a chunk size, ForwardDiff will try to guess one for you
+If you do not explicitly provide a chunk size, ForwardDiff will try to guess one for you
 based on your input vector:
 
 ```julia