Fix typos

On unrelated note, why on earth check like in safeAdd? Is it actually an overflow check?

docs/source/safemath.rst

@@ -13,12 +13,12 @@ safeMul(uint a, uint b) internal returns (uint)
 
 Multiplies two unisgned integers. Asserts that dividing the product by the non-zero multiplicand results in the multiplier.
 
-safeSub(uint a, unit b) internal returns (uint)
+safeSub(uint a, uint b) internal returns (uint)
 """""""""""""""""""""""""""""""""""""""""""""""""
 
 Checks that b is not greater than a before subtracting.
 
-safeAdd(unit a, unit b) internal returns (uint)
+safeAdd(uint a, uint b) internal returns (uint)
 """""""""""""""""""""""""""""""""""""""""""""""""
 
 Checks that the result is greater than both a and b.