Add a missing period. (#2604)

Author: GunnarFarneback

src/manifest.jl

@@ -283,7 +283,7 @@ end
 function write_manifest(manifest::Manifest, manifest_file::AbstractString)
     if manifest.manifest_format.major == 1
         @warn """The active manifest file at `$(manifest_file)` has an old format that is being maintained.
-            To update to the new format run `Pkg.upgrade_manifest()` which will upgrade the format without re-resolving""" maxlog = 1
+            To update to the new format run `Pkg.upgrade_manifest()` which will upgrade the format without re-resolving.""" maxlog = 1
     end
     return write_manifest(destructure(manifest), manifest_file)
 end