kevm-pyk/evm: correct variable name

Author: ehildenb

kevm-pyk/src/kevm_pyk/kproj/evm-semantics/evm.md

@@ -389,7 +389,7 @@ The `#next [_]` operator initiates execution by:
     rule #stackUnderflow( _W0 : _W1 : _W2 : _W3 :                         _WS , _QOP:QuadStackOp ) => false
     rule #stackUnderflow( _W0 : _W1 : _W2 : _W3 : _W4 : _W5 : _W6 :       _WS , _CSOP:CallSixOp  ) => false
     rule #stackUnderflow( _W0 : _W1 : _W2 : _W3 : _W4 : _W5 : _W6 : _W7 : _WS , COP:CallOp       ) => false requires notBool isCallSixOp(COP)
-    rule #stackUnderflow(WS, _POP:OpCode) => #sizeWordStack(WS) <Int #stackNeeded(OP) [owise]
+    rule #stackUnderflow(WS, OP:OpCode) => #sizeWordStack(WS) <Int #stackNeeded(OP) [owise]
 
     rule #stackOverflow(_WS, OP) => false requires #stackDelta(OP) <=Int 0
     rule #stackOverflow(WS, OP) => 1024 <Int #sizeWordStack(WS) +Int #stackDelta(OP) [owise]