Fix building, RottenLinks extension renamed master branch to main (#489)

Author: pastakhov

Dockerfile

@@ -455,7 +455,7 @@ RUN set -x; \
 	&& cd $MW_HOME/extensions/RevisionSlider \
 	&& git checkout -q 3cae51a322a5ca0f359e83efcb5fac38e73e346e \
 	# RottenLinks
-	&& git clone --single-branch -b master https://github.com/miraheze/RottenLinks.git $MW_HOME/extensions/RottenLinks \
+	&& git clone --single-branch -b main https://github.com/miraheze/RottenLinks.git $MW_HOME/extensions/RottenLinks \
 	&& cd $MW_HOME/extensions/RottenLinks \
 	&& git checkout -q a96e99d0a61a42d59587a67db0720ce245a7ee46 \
 	# SandboxLink