use sh syntax insteda of bash

Author: sebsto

Makefile

@@ -12,7 +12,7 @@ generate-docs:
 	# Dynamically add the swift-docc-plugin for doc generation
 	cp Package.swift Package.swift.bak
 	for manifest in Package.swift Package@*.swift ; do \
-		if [[ -f "$$manifest" ]] && ! grep -E -i "https://github.com/(apple|swiftlang)/swift-docc-plugin" "$$manifest" ; then \
+		if [ -f "$$manifest" ] && ! grep -E -i "https://github.com/(apple|swiftlang)/swift-docc-plugin" "$$manifest" ; then \
 			echo "package.dependencies.append(" >> "$$manifest" ; \
 			echo "	.package(url: \"https://github.com/swiftlang/swift-docc-plugin\", from: \"1.4.5\")" >> "$$manifest" ; \
 			echo ")" >> "$$manifest" ; \