docs: typo 'These operators modifies' -> 'These operators modify'. (#918)

Author: heijp06

concepts/numbers/about.md

@@ -60,7 +60,7 @@ Consider also that we are only assigning the value of `width` to `length` at the
 Therefore, if the value of `width` changes at a later moment, it will not affect the value taken by `length`.
 
 Assignment operator can be combined with the other operators(arithmetic & bitwise) known as `compound assignment` operators `+=`, `-=`, `*=`, `/=`, `%=`.
-These operators modifies the current value of a variable by performing an operation on it.
+These operators modify the current value of a variable by performing an operation on it.
 
 ```cpp
 // we start with 0 people

concepts/numbers/introduction.md

@@ -60,7 +60,7 @@ Consider also that we are only assigning the value of `width` to `length` at the
 Therefore, if the value of `width` changes at a later moment, it will not affect the value taken by `length`.
 
 Assignment operator can be combined with the other operators(arithmetic & bitwise) known as `compound assignment` operators `+=`, `-=`, `*=`, `/=`, `%=`.
-These operators modifies the current value of a variable by performing an operation on it.
+These operators modify the current value of a variable by performing an operation on it.
 
 ```cpp
 // we start with 0 people

exercises/concept/freelancer-rates/.docs/introduction.md

@@ -60,7 +60,7 @@ Consider also that we are only assigning the value of `width` to `length` at the
 Therefore, if the value of `width` changes at a later moment, it will not affect the value taken by `length`.
 
 Assignment operator can be combined with the other operators(arithmetic & bitwise) known as `compound assignment` operators `+=`, `-=`, `*=`, `/=`, `%=`.
-These operators modifies the current value of a variable by performing an operation on it.
+These operators modify the current value of a variable by performing an operation on it.
 
 ```cpp
 // we start with 0 people