Rename aliases directive to alias

Refs #9562

Code/Mantid/docs/sphinxext/mantiddoc/directives/aliases.py → Code/Mantid/docs/sphinxext/mantiddoc/directives/alias.py

 from base import BaseDirective
 
 
-class AliasesDirective(BaseDirective):
+class AliasDirective(BaseDirective):
 
     """
-    Obtains the aliases for a given algorithm based on it's name.
+    Obtains the alias for a given algorithm based on it's name.
     """
 
     required_arguments, optional_arguments = 0, 0
 
     def run(self):
         """
-        Called by Sphinx when the ..aliases:: directive is encountered.
+        Called by Sphinx when the ..alias:: directive is encountered.
         """
         alg = self.create_mantid_algorithm(self.algorithm_name(), self.algorithm_version())
         alias = alg.alias()
         if len(alias) == 0:
             return []
 
-        self.add_rst(self.make_header("Aliases"))
+        self.add_rst(self.make_header("Alias"))
         format_str = "This algorithm is also known as: **%s**"
         self.add_rst(format_str % alias)
         self.commit_rst()
@@ -32,4 +32,4 @@ def setup(app):
     Args:
       app: The main Sphinx application object
     """
-    app.add_directive('aliases', AliasesDirective)
+    app.add_directive('alias', AliasDirective)