rpn-calculator: replace tabs with spaces

Author: petertseng

exercises/concept/rpn-calculator/.docs/instructions.md

@@ -106,8 +106,8 @@ pub enum CalculatorInput {
 
 pub fn evaluate(inputs: &[CalculatorInput]) -> Option<i32> {
     unimplemented!(
-		"Given the inputs: {:?}, evaluate them as though they were a Reverse Polish notation expression",
-		inputs,
-	);
+        "Given the inputs: {:?}, evaluate them as though they were a Reverse Polish notation expression",
+        inputs,
+    );
 }
 ```

exercises/concept/rpn-calculator/src/lib.rs

@@ -9,7 +9,7 @@ pub enum CalculatorInput {
 
 pub fn evaluate(inputs: &[CalculatorInput]) -> Option<i32> {
     unimplemented!(
-		"Given the inputs: {:?}, evaluate them as though they were a Reverse Polish notation expression",
-		inputs,
-	);
+        "Given the inputs: {:?}, evaluate them as though they were a Reverse Polish notation expression",
+        inputs,
+    );
 }