Fix sed for better portability

scripts/compare.sh

@@ -1,6 +1,7 @@
 #!/bin/bash
 cargo build --release
-sed -i '' '/## Benchmarks/,$d' README.md
+sed -i.bak '/## Benchmarks/,$d' README.md
+rm README.md.bak
 
 CORES=$(sysctl -n hw.ncpu 2>/dev/null || grep -c ^processor /proc/cpuinfo)
 CPU=$(sysctl -n machdep.cpu.brand_string 2>/dev/null || grep '^model name' /proc/cpuinfo | sed 's/model name.\+: //g' | head -n 1)
@@ -9,7 +10,7 @@ OPTIPNG_VERSION=$(optipng -v | head -n 1)
 RUST_VERSION=$(rustc -V)
 echo -e '## Benchmarks\n' >> README.md
 echo "Tested $OXIPNG_VERSION (compiled on $RUST_VERSION) against $OPTIPNG_VERSION on $CPU with $CORES logical cores" >> README.md
-echo -e '\n\n```\n' >> README.md
+echo -e '\n```\n' >> README.md
 
 hyperfine --style basic --warmup 5 './target/release/oxipng -P ./tests/files/rgb_16_should_be_grayscale_8.png' 'optipng -simulate ./tests/files/rgb_16_should_be_grayscale_8.png' >> README.md
 echo -e '\n\n' >> README.md