pluginupdate.py: fix regression with plugin line splitting

Before 46c68ad both "@" and " as " could be used in the same line like
the following:

    vimwiki/vimwiki@dev as vimwiki-dev

After 46c68ad this gives an error due to the split URI still erroneously
including the " as [name]" at the end, due to splitting from the wrong
variable.

maintainers/scripts/pluginupdate.py

@@ -470,10 +470,10 @@ def parse_plugin_line(line: str) -> PluginDesc:
     alias = None
     uri = line
     if " as " in uri:
-        uri, alias = line.split(" as ")
+        uri, alias = uri.split(" as ")
         alias = alias.strip()
-    if "@" in line:
-        uri, branch = line.split("@")
+    if "@" in uri:
+        uri, branch = uri.split("@")
 
     repo = make_repo(uri.strip(), branch.strip(), alias)