Update README.md (#901)

Incorrect description for `P`: steps to previous position, not instruction

Author: riordant

src/hevm/README.md

@@ -40,7 +40,7 @@ Note: some `hevm` commands (`dapp-test`) assume the use of the `ds-test` framewo
 - `0`: choose the branch which does not jump
 - `1`: choose the branch which does jump
 - `N`: step to the next source position
-- `P`: step previous source instruction
+- `P`: step previous source position
 - `C-n`: step to the next source position and don't enter `CALL` or `CREATE`
 - `C-p`: step previous source position without entering
 - `m`: toggle memory view