Removed aria2c.rst from the prerequisites of $(HTML)

$(HTML) is the root directory of html documents and since we don't
remove $(HTML) when re-building htmls, it is useless to use aria2c.rst
as a prerequisites.

doc/manual-src/en/Makefile.am

@@ -34,7 +34,7 @@ dist-hook:
 $(man_MANS): aria2c.rst
 	$(MAKE) man
 
-$(HTML): aria2c.rst
+$(HTML):
 	$(MAKE) html
 
 # Autogenerated rules by sphinx-quickstart

doc/manual-src/ru/Makefile.am

@@ -35,7 +35,7 @@ dist-hook:
 $(man_MANS): aria2c.rst
 	$(MAKE) man
 
-$(HTML): aria2c.rst
+$(HTML):
 	$(MAKE) html
 
 # Autogenerated rules by sphinx-quickstart