Fix the heed3 script on Linux

Author: Kerollmops

convert-to-heed3.sh

@@ -14,7 +14,11 @@ cp heed3/Cargo.toml heed/Cargo.toml
 
 # ...and replaces the `heed::` string by the `heed3::` one.
 for file in $(find heed/src -type f -name "*.rs"); do
-    sed -i '' 's/heed::/heed3::/g' "$file"
+    if [[ "$OSTYPE" == "darwin"* ]]; then
+        sed -i '' 's/heed::/heed3::/g' "$file"
+    else
+        sed -i 's/heed::/heed3::/g' "$file"
+    fi
 done
 
 echo "Heed3 crate setup completed successfully. Copied and modified configurations for the heed crate."