Include reference in CoerceVariableValues

Author: leebyron

spec/Section 6 -- Execution.md

@@ -77,8 +77,8 @@ the operation fails without execution.
 CoerceVariableValues(schema, operation, variableValues):
 
 - Let {coercedValues} be an empty unordered Map.
-- Let {variableDefinitions} be the variables defined by {operation}.
-- For each {variableDefinition} in {variableDefinitions}:
+- Let {variablesDefinition} be the variables defined by {operation}.
+- For each {variableDefinition} in {variablesDefinition}:
   - Let {variableName} be the name of {variableDefinition}.
   - Let {variableType} be the expected type of {variableDefinition}.
   - Assert: {IsInputType(variableType)} must be {true}.