Refactored string_set_at to be immutable and added String documentation

docs/vector.md

@@ -5,7 +5,7 @@ aspects (internal design, memory layout, etc.) of the `Vector` data structure.
 `Vector` is a dynamic array with generic data type support; this means that you can store
 any kind of homogenous value on this data structure. Resizing is performed automatically
 by increasing the capacity by 1.5 times when the array becomes full. Internally, this 
-data structure is represented by the following structure:
+data structure is represented by the following layout:
 
 ```c
 typedef struct {
@@ -39,7 +39,7 @@ At the time being, `Vector` supports the following methods:
 - `size_t vector_size(vector)`: return vector size (i.e., the number of elements);  
 - `size_t vector_capacity(vector)`: return vector capacity (i.e., vector total size).
 
-As you can see by the previous function signatures, most methods that operate
+As you can see from the previous function signatures, most methods that operate
 on the `Vector` data type return a custom type called `vector_result_t` which is
 defined as follows:
 
@@ -66,7 +66,7 @@ Each method that returns such type indicates whether the operation was successfu
 by setting the `status` field and by providing a descriptive message on the `message`
 field. If the operation was successful (that is, `status == VECTOR_OK`), you can either
 move on with the rest of the program or read the returned value from the sum data type. Of course, you can choose to 
-ignore the return value (if you're brave enough :D) as illustrated in the first part of the README.
+ignore the return value (if you're brave enough :D) as illustrated on the first part of the README.
 
 ## Functional methods
 `Vector` provides three functional methods called `map`, `filter` and `reduce` which allow the caller to apply a computation to the vector,