Fix documentation reference to local_manifest.xml

Documentation of the remove-project element still refers explicitly
to local_manifest.xml.

Change it to the more generic "a local manifest".

Change-Id: I6278beab99a582fae26a4e053adc110362c714c2

docs/manifest-format.txt

@@ -209,7 +209,7 @@ Deletes the named project from the internal manifest table, possibly
 allowing a subsequent project element in the same manifest file to
 replace the project with a different source.
 
-This element is mostly useful in the local_manifest.xml, where
+This element is mostly useful in a local manifest file, where
 the user can remove a project, and possibly replace it with their
 own definition.