Edit Unit Testing Documentation (#8961)

docs/unit_testing.md

@@ -50,7 +50,7 @@ In that model you would emulate the input, and expect a certain output from the
 
 # Tracing Variables :id=tracing-variables
 
-Sometimes you might wonder why a variable gets changed and where, and this can be quite tricky to track down without having a debugger. It's of course possible to manually add print statements to track it, but you can also enable the variable trace feature. This works for both for variables that are changed by the code, and when the variable is changed by some memory corruption.
+Sometimes you might wonder why a variable gets changed and where, and this can be quite tricky to track down without having a debugger. It's of course possible to manually add print statements to track it, but you can also enable the variable trace feature. This works for both variables that are changed by the code, and when the variable is changed by some memory corruption.
 
 To take the feature into use add `VARIABLE_TRACE=x` to the end of you make command. `x` represents the number of variables you want to trace, which is usually 1.