Fix typo in docc dockerfile

Author: shahmishal

swift-ci/swift-docc-render/build-swift-docc-render.sh

@@ -70,7 +70,7 @@ docker build -t swift-docc-render:latest \
   --build-arg SWIFT_DOCC_RENDER_BRANCH="$BRANCH" \
   --build-arg http_proxy="$http_proxy" \
   --build-arg https_proxy="$https_proxy" \
-  --build-arg no_proxy="$no_proxy \
+  --build-arg no_proxy="$no_proxy" \
   -f "$DOCKERFILE_PATH" \
   "$DIRECTORY_ROOT"