Fix Phragmen benchmark (#11137)

* Fix phragmen benchmark

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* cargo run --quiet --profile=production  --features=runtime-benchmarks --manifest-path=bin/node/cli/Cargo.toml -- benchmark --chain=dev --steps=50 --repeat=20 --pallet=pallet_elections_phragmen --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --output=./frame/elections-phragmen/src/weights.rs --template=./.maintain/frame-weight-template.hbs

Co-authored-by: Parity Bot <admin@parity.io>